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Table of Contents Preface

PLCs in a glance 07

Basic PLC Operation 07

1- Processor Operating Cycle 07

2- Input modules 08

3- Output modules 08

4- Programming device 08

Ladder Diagrams 09

Hard-wired Control 10

Replacing hard-wired control with a PLC 11

The advantage of using a PLC as a controller 11

Different PLC models made by Siemens 12

1- SIMATIC S5 12

2-SIMATIC S7 12

3-LOGO! 13

MicroLogix 1500 PLC 13

Number system – decimal & binary 14

Logic 0, Logic 1 15

Actuator 16

Discrete Inputs 16

Analog Inputs 18

Discrete Outputs 18

Analog outputs 18

CPU (Central Processing Unit) 19

Programming Languages 19

Ladder Logic programming 20

Reading Ladder Logic Diagrams 21

Function Block Diagrams (FBD) 22

Hardware 23

PLC memory size 23

1- RAM/ ROM / EPROM / Firmware 23

2- Memory structure 24

3- Program space 24

4- Data space 24

Software 25

Cables 26

S7-200 & A-B MicroLogix 1500 PLCs 26

PLC models 27

Optional cartridge 28

Expansion modules 28

Understanding Controller Status indicators 28

Using a DIN rail to Install PLC and Expansion units 29

External power supplies 30

I/O numbering 31

Inputs / Outputs 32

Super Capacitor 32

PLC Display and HMI units 32

Siemens TD200 33

Computer network 33

PROFIBUS connection 33

Powerful AS-Interface connection 33

Contact & coil symbols (Ladder format) 34

Input/Output & contact programming examples 36

Functions mostly used in PLC programming 38

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1- AND function 38

2- OR function 39

3- Latch function 40

4- Inverse function 40

FBD (Function Block Diagram) programming language 41

STL (Statement List) programming language 41

Normally Open (NO) & Closed Contacts (NC) 42

Boolean Algebra PLC programming 43

Status Function 45

Force Function 46

Sequence 47

3 phase motor starter program 48

Wiring motor Starter circuit with PLC 50

Expanding the previous problem 51

Introduction to Analog signals 53

Analog Output signals 54

Introduction to A-B & Siemens S7-200 Timers 55

Hard-wired time Delay relay 56

A-B MicroLogix 1500 & SIMATIC S7-200 Timers 56

1-Timer On-Delay (TON) 56

2- Timer Off-Delay (TOF) 56

3- Timer Retentive On-Delay Timer (RTO) 56

An On-Delay timer programming examples & application 57

A-B Off-Delay timer 57

An Off-Delay (TOF) application example 58

A-B Retentive On-Delay Timer programming 59

SIEMENS SIMATIC S7-200 timers 60

1- SIMATIC On-Delay timer application example 61

2-How to calculate PT? 61

3- S7-200 Off-Delay timer (ladder Logic symbol and functions) 61

4- S7-200 Retentive On-Delay timer (ladder Logic symbol and functions) 62

A-B SLC & Siemens S7-200 Counters 63

1-Up-counter (CTU) definition, symbol & an application example 64

2- Down-counter (CTD) definition and symbol& and application example 66

SIEMENS SIMATIC S7-200 Counters 66

1-Regular Counters 66

A- CTUD ~ Count Up/Down counter 66

B-CTU ~ Count Up counter 66

C- CTD ~ Count Down counter 66

2- High-Speed Counters 66

Sample problem 68

Pulse Train Output (PTO) function 69

Pulse Width Modulation (PWM) function 70

Transmit function 71

Introduction to Network Communication 72

AS-Interface (AS-i) the Actuator Sensor Interface 72

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PLCs in a glance

A PLC or a Programmable Logic Controller, is a programmable controller which

is considered as a member of computer family is mainly designed to be used in an

industrial field to sense different incoming signals, process them (or make decision) and

issue commands based on the special software program in its memory. This tutorial guide

is designed to supply you with basic information on the functions and configurations of

PLCs which is especially focused on Siemens S7-200 and Allen Bradley MicroLogix

1500.

Figure 1 illustrates Some I/O field devices connected to a Siemens S7-200 PLC

(Courtesy of Siemens Industrial Automation)

Basic PLC Operation

Processor Operating Cycle ~ in general

All hardware associated with a PLC falls into one of two functional areas. The

actual intelligence of the PLC is derived from electronic computer-based hardware,

which comprises the processor, or CPU, portion of the system as represented in

figure 2. The processor section of a PLC includes a power supply, a microprocessor or

special-purpose electronic circuitry, as well as a computer-type memory for the storage of

programming instructions and system data. All activity of the PLC system is handled by

the processor. The processor is responsible for the analysis of incoming as well as

previously stored data, and for responding to that information according to a detailed

control plan stored within the unit by the user.

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Most PLC systems offer the standard relay, latch, timing, counting, and simple

mathematical functions of addition, subtraction, multiplication, and division as part of

their capabilities.

Figure 2

From figure 2, notice that a typical PLC system can be divided into four sections:

1-Programming Device

2-Micrrocessor + Memory Unit

3-Power Supply

3-Input/Output Sections

Input modules accept a variety of analog or digital signals from various sensors and

convert them to logic signals that can be used by the CPU.

The CPU makes decisions and executes control instructions based on the program in its

memory.

Output modules convert control instructions from the CPU into signals that can be used

to control various field devices

A programming device is used to install the instructions that determine what the PLC

will do in response to specific inputs

Processor Operating Cycle ~ in general

During each operating cycle, the PLC processor does the following continually:

1-Reads current input module statuses and updates Input Image Table.

2-PLC processor continually solves user Logic program based on current

Input Image Table statuses

3- PLC’s processor updates Output Images Table statuses based on solution of user

Logic Program.

4- Based on the result on step # 3, PLC processor continually activates or deactivates

I/O module status according to Output Image Table status.

And then goes to the next rung. But if it is the end of the ladder logic program, and the

last statement is “END” instruction, it does some other tasks such as communication and

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housekeeping tasks and then goes back to step 1 and continues executing ladder logic

program for the second time. It does so over and over until the time that the PLC is taken

off line or shut down manually.

Based on what is said, notice that a single operating cycle or scan which is illustrated in

figure 3, can be divided into two distinct parts- the I/O scan and the program scan.

Figure 3

Basic PLC Operation

Ladder Diagrams

Ladder diagrams are specialized schematics commonly used to document

industrial control logic systems. They are called “ladder diagrams” because they resemble

a ladder, with two vertical rails (supply power) and as many as rungs (horizontal lines) as

there are control circuits to represent. If we want to draw a simple ladder diagram

showing a lamp that is controlled by a hand switch, it would look like this:

Figure 4 displays a simple ladder diagram without power supply shown

The L1 and L2 designations refer to the two poles of a 120 VAC supply, unless

otherwise noted, L1 is the hot conductor, and L2 is the ground (neutral) conductor.

Typically in industrial relay logic circuits, but not always, the operating voltage

for the switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even

DC systems are sometimes built and documented according to Ladder diagrams.

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Ladder diagrams are used to describe the logic of electrical control systems.

There are differences in the way ladder logic was implemented in computerized form as

compared to hard-wired. The basic component of the control system is the control relay

which is a solenoid that operates a number of switches or contacts. The contacts come

normally open and normally closed, normal being when the relay is not energized. Relays

come in various breeds like time delay and latching types. Other components of the

control system are the field devices such as push buttons, limits switches, lights, and

controlled devices like motor starters and solenoid operated valves.

Figure 5 displays a standard motor control circuit (Start /Stop Circuit)

O.L

MM

2 3

3 ~mo t o r

L 1 L 2 L 3 N

M M M

Figure 6 displays Hard-Wired Control of an actual 3 phase motor

with the controlling circuit on the right side of the figure

Hard-Wired Control

Prior to invention of PLCs, many control tasks were done with contactors and

relays hard-wired together. Circuits first had to be designed and drawn up. Then

components were specified and installed, and wiring list is created. Electricians would

then wire it all together. If something went wrong, the designers and electricians had to

rework the installation. If changes were made later, they could be time-consuming and

expensive. PLCs can perform the same tasks as hard-wired controls, and more complex

functions as well. The connections between field devices and relay contacts take place in

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the PLC instead of with external wiring. Hard wiring, though still required to connect

field devices, is less extensive. Installations are easier to modify, since this often involves

changing only the PLC program.

Replacing hard-wired control with a PLC

Now what we would like to do is to replacing control circuit in figure 5 with a

PLC and use special program to control start / stop function of the 3 phase electro motor

later. Of course nobody uses a PLC just to control start / stop function. This example is

only for educational purposes. See figure 7.

Figure 7 illustrates a typical PLC connected to input

& output field devices.

The advantage of using a PLC as a controller is

1- controlling circuitry is smaller

2- ease of troubleshooting

3- less need to repair

4- a plc can execute much complicated functions in the controlling systems

5- A plc can easily establish communication with other process control systems

The all above advantages caused the PLCs to win the war against the old way of

doing things with hardwired circuitry using bulky relays, timers and other field devices.

In short, the first PLCs were invented on 1968, and in year 1970 communicating circuitry

was added to them. In 1980 communication protocols were standardized and eventually,

in year 1990, programming languages were standardized (IEC1131).

IEC1131 standard

In year 1979, a group of experts under the name IEC (International Electro-

technical Commission) gathered to investigate ways to standard PLC parameters as long

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as the hardware, software programming language and communication systems are

concerned. These studies and research took about 12 years and finally IEC1131 standard

was formed. IEC1131 standard is simply rules and regulations which must be considered

by PLC manufactures and cover different aspects of PLCs. IEC1131 suggests the

specifications set forward as long as the hardware design, programming language,

troubleshooting of hardware, installation and testing is concerned.

Different PLC models made by Siemens

Siemens categorizes its range of PLCs under the name SIMATIC ®. Some of

these PLCs are built in Compact form meaning the PLC box contains power supply,

CPU, input and output models. All these different part are located in plc box and

considered as a unit. On the other hand, the second model of PLC is built in Modular

form. In this form of application, the end user can choose which model he needs for his

particular need.

SIMATIC S5

These PLCs which are relatively older version of Siemens models. Some of the

models were built is compact form such as S5-90U or S5-95U with limited range of

application and functionality. The other models, such as S5-100U or S5-115U were built

in Modular form which could be used to control middle range of application. For broader

range of application, models S5-155U and 135U could be used from the same family.

Siemens STEP 5 software is used to program SIMATIC S5 PLCs.

Figure8 illustrates few models of

SIEMENS SIMATIC S5 ® models of PLCs

(Courtesy of Siemens Industrial Automation)

SIMATIC S7 PLCs

SIMATIC S7 PLCs are next generation of controllers built after S5 series. S7

series come in three different models which are: S7-200 (compact), S7-300 (modular)

and S7-400 (also modular). S7-200 is used to control a relatively small system, The S7-

200 is ideal for smaller stand-alone applications such as elevators, car washes, or mixing

machines. It can also be used to advantage with more complex industrial applications,

such as bottling and packaging machines.

S7-300 for mid-range applications and finally S7-400 can be used for broader range of

control systems. These applications require a greater number of I/O points. Both the 300

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and 400 are modular and expandable. The power supply and I/O functions are contained

in separate modules that connect to the CPU module. Choosing between the S7-300 and

S7-400 depends on the complexity of the task and on possible future expansion.

Figure 9 illustrates S7-300 and S7-400 PLCS

(Courtesy of Siemens Industrial Automation)

Logo! Logic Modules

SIMATIC S7-200 ® & LOGO! ® are both inexpensive and simple controllers

which are used for small control projects (inside of buildings or small machineries).

LOGO is a compact PLC which is programmed by the keyboard on its panel or one can

use LOGO! Soft Comfort ® software to program it via a PC and download the ladder

logic program into the LOGO! PLC’s memory. S7-200 micro PLC is offered by five

different CPUs that can be expanded with a wide range of individual modules.

Programming is based on the easy to use software STEP- 7 Micro/WIN ®. Hence, the

SIMATIC S7-200 is a reliable, fast and flexible controller in the field of micro

automation. Figures 10 shows picture of both controllers and 11 MicroLogix 1500 with

two side I/O modules on the right.

Figure 10 illustrates LOGO! and S7-200 PLCs

(Courtesy of Siemens Industrial Automation)

MicroLogix 1500 PLC

Bulletin 1764 MicroLogix controllers are the most expandable members of the

MicroLogix family which are made by Allen-Bradley. This controller fits many

applications that traditionally called for larger and more expensive controllers.

MircoLogix 1500 is a compact controller which means a processor, base unit with power

supply and embedded I/O, all are packed inside of the controller box. This controller

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packs the best features of a modular system into an inexpensive, small footprint. Figure

11 illustrates a MicroLogix 1500 PLC with two extra I/O modules connected to it.

Figure 11 illustrates MicroLogix 1500®

(Courtesy of Allen Bradley)

Number Systems – Decimal & Binary

Everywhere, except for computer-related operation, the main system of

mathematical notation today is the decimal, which is a based”10” system. As in other

systems, the position of a symbol in a base”10” number denotes the value of that symbol

in terms of exponential values of the base. That is, in the decimal system, the quantity

represented by any of the ten symbols used: 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9, depends on its

position in the number. Unlike the decimal system, only two digits “0” and “1” suffice to

represent a number in the binary system. The binary system plays a crucial role in

computer science and technology.

Flip-flops-electronic devices that can only carry two distinct voltages at their outputs and

that can be switched from one state to the other state by an impulse signal, which can also

be used to represent binary numbers; the two voltages correspond to the two digits.

Optical and magneto-optical storage devices use two distinct levels of light reflectance or

polarization to represent 0 or 1.

Hence, arithmetic operations in the binary system are extremely simpler than doing it in

decimal system as long as computer hardware design is concerned.

Since a PLC is a computer, it stores information in the form of 1 = High (or ON) and 0 =

Low (OFF), conditions (1 or 0), referred to as binary digits or bits. Sometimes single bits

are used to represent ON and OFF conditions. At other times they are combined to

represent numerical values. All number systems have three characteristics: digits, base,

and weight. The decimal system, based on the number 10, has these characteristics:

Ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9

Base: 10

Weights: 1, 10, 100, 1000,…(powers of base 10)

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The binary system which is used by PLCs, it also has these characteristics:

Two digits: 0, 1

Base: 2

Weights: 1, 2, 4, 8, 16, 32, 64, 128, …(powers of base 2)

Logic 0, Logic 1

PLCs use both digital and analog signals, but the CPU itself can only understand

digital signals. These signals are either ON or OFF. The binary number system is used to

represent digital signals, since binary numbers can be represented with only two digits,

that is 1 or ON and 0 or OFF. Binary 1 indicates that a signal is present, or a switch is

ON. Binary 0 indicates that the signal is not present, or the switch is OFF.

Figure 12

Sensors

A Sensor is a device, which responds to an input quantity by generating a

functionally related output usually in the form of an electrical or optical signal for use by

PLCs. Sensors are connected to the inputs of PLCs. One example is a pushbutton. An

electrical signal is sent from the pushbutton to a PLC input, indicating the condition of

the pushbutton’s contacts which is either un-pressed or pressed (un-activated or

activated).

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Figure 13

Actuator

An actuator is a transducer that accepts a signal and converts it to a physical

action. In other words, an actuator causes an action to occur relating to the data that was

sent to it. Actuators convert electrical signals from PLC outputs into physical conditions.

in our case, we use contactors as “actuators” which are activated any time PLC’s output

terminal is “1 or ON” and un-activated when output terminal is “0 or OFF”.

Figure 14

Discrete Inputs

Discrete inputs are inputs to a PLC that require an on or off signal. Pushbuttons,

toggle switches, limit switches, proximity switches, and contact closures are examples of

discrete sensors. These discrete sensors may be connected to PLC discrete inputs. In the

ON condition, the state of a discrete input may be referred to as a logic 1 or “ON” or

“High”. In the OFF condition it is referred to as logic 0, or low.

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Figure 15 discrete input examples – momentary contact / Normally

Open and close Pushbutton switches (NO or NC)

Normally open switches are the type used to turn on or off when it is pressed or

un-pressed accordingly. When the switch is pressed, the circuit becomes closed and

functions. In the following figure, when the switch is not depressed, no voltage is present

at the PLC input. This is the OFF condition. When the button is depressed, 24 VDC is

applied to the PLC input. This is the ON condition. Hence, normally closed switches

perform just the opposite. When the switch is pressed, the circuit becomes open.

Figure 16

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Analog Inputs

While a digital input can tell us about discrete changes in the physical world, such

as whether a lamp is on or off, there are times when this is not enough. Sometimes we

want to know how much is the weight of a load “placed on” a Force- sensitive resistors

(FSRs) that change resistance based on a changing force applied to the surface of the

sensor; thermistors that change resistance in response to changing heat; and many more.

To measure varying signals we need to have analog inputs. Typical analog inputs vary

from 0 to 20 milliamps, or 0 to 10 volts. In the accompanying example, a level

transmitter monitors the level of liquid in a tank. Depending on the type of level

transmitter, the voltage on the PLC input either increases or decreases as the liquid level

increases.

Figure 17

Discrete Outputs

The word “discrete” means a signal that has two states, ON and OFF. Therefore, a

discrete output, or a digital output, is either ON or OFF. Solenoids, contactor cols, and

lamps are examples of actuator devices normally connected to discrete outputs. In the

accompanying example, the lamp can be turned on or off by the PLC output.

Figure 18

Analog Outputs

The word “analog” means a continuously variable signal. Analog signals differ

from digital signals in that small fluctuations in the analog signal are meaningful. Hence,

an analog output signal varies continuously. The analog signal voltage level might be as

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simple as a 0-10 VDC that drives an analog meter. Examples of analog meter outputs are

speed, weight, and temperature. The output signal may also be used on more complex

applications such as a current to pneumatic transducer that controls an air-operated flow-

control valve.

Figure 19

CPU

PLCs are often defined as miniature industrial computers that contain hardware

and software that is used to perform control functions. A PLC consists of two basic

sections: the central processing unit (CPU) and the input/output interface system. The

CPU, which controls all PLC activity, can further be broken down into the processor and

memory system. The input/output system is physically connected to field devices

(switches, sensor, etc) and provide the interface between the CPU and the information

provider (inputs) and controllable devices (outputs). To operate, The CPU monitors the

inputs and makes decisions based on instructions held in its program memory. It performs

relay, counting, timing, data comparison, and sequential operations.

Programs are typically created in ladder logic, a language that closely resembles a relay-

based wiring schematic, and are entered into the CPU’s memory prior to operation and

changes only when a change is made to the control program.

Figure 20

Programming Languages

In short, when it comes to PLC programming languages, it defines 5 different

types of data which can be used by the programming languages. IEC1131 standards six

different programming languages for PLCs.

1- IL (Instruction List)

2- FBD (Function Block Program)

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3- LD (Ladder Diagram)

4- ST (Structured Text)

5- SFC (Sequential Function Control)

6- CFC (Continuous Function Chart)

Figure 21 illustrates a sample of each programming language

Ladder Logic Programming

Ladder logic is the main programming method used for PLCs. Ladder logic has

been developed to mimic relay logic. The decision to use the relay Logic diagrams was a

strategic one. By selecting ladder logic as the main programming method, the amount of

re-training needed for engineers and trades people was greatly reduced. The reason it’s

called “Ladder logic” is the program is drawn pictorially and looks like a ladder.

There are several techniques used to look at and understand programs. Among them are

ladder logic, statement lists, and function block diagrams. Ladder logic programming

uses components that resemble elements used in line diagram. Line diagrams are used to

describe hard-wired control systems. The accompanying diagram is an example of a

ladder logic program.

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Figure 22

Reading Ladder Logic Diagrams

In a ladder logic program, the left vertical line represents the power of energized

conductor. The other end of all output elements or instructions represent the neutral or

return circuit path (the right vertical line that represents the neutral on hard-wired

diagrams is omitted). Ladder logic diagrams are read from left to right and from top to

bottom. The ladder’s rungs are referred to as networks. A network may have several

control elements, but only one output coil.

Figure 23

Ladder Logic and Statement List (or Instruction List)

I0.0, I0.1, and Q0.0 are the first instruction combination located in the first rung.

It means If inputs I0.0 AND I0.1 are energized, output relay Q0.0 energizes.

In the second instruction combination, means if either I0.4 OR I0.5 is energized, output

relay Q0.1 will be energized.

A statement list is another way of viewing a program. The operations are shown on the

left. The operands are shown on the right. Comparing the statement list and ladder logic

diagram shows that they have a similar structure, and define the same program.

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Figure 24

Function Block Diagrams (FBD)

Function Block Diagrams provide another way to develop PLC control

programs. Each function has a name to designate its specific task, and is indicated by a

rectangle with its function name inside. Inputs are shown on the left and outputs on the

right. The accompanying function block diagram performs the same function as the

previous ladder logic diagram and statement list.

Figure 25

Software

PLC programs are typically written in a special application on a personal

computer, then it is downloaded by a direct-connection cable or over a network to the

PLC. The program is stored in the PLC either in battery-backed-up RAM or some other

non-volatile flash memory. Each PLC manufacture designees a special type of

programming software that can be used by end users to develop PLC control programs.

Often, a single PLC can be programmed to replace thousands of relays. As an example,

Siemens has developed S7 software for the S7 family of PLC. To program PLCs made by

Allen Bradley one might need to purchase software developed by Rockwell Automation.

Although in some cases, third party companies also developed and market software for

other manufactures brand of PLCs.

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Hardware

Hardware is the actual equipment. The PLC, the Programming device, and the

connecting cable are examples of hardware.

Figure 26

PLC Memory Size

Memory in a PLC system is divided into the program memory which is usually

stored in EPROM/ROM, and the operating memory. The RAM memory is necessary for

the operation of the program and the temporary storage of input and output data. Typical

memory sizes of PLC systems are around 1kb (when talking about computer or PLC

memory, 1 k refers to 1024 units) for small PLCs. This can be 1024 bits, 1024 bytes, or

1024 words, depending on memory type. Few kb for medium sizes and greater than 10-

20 kb for larger PLCs depending on the requirements. Many PLC would support easy

memory upgrades fro future expansions.

While it is common for PLCs to measure their memory capacity in words, it is important

to know the number of bits in each word. A PLC that uses 8-bit words would have half

the memory capacity of a PLC that uses 16-bit words. For example a PLC that uses 8-bit

words with an 8K word capacity has 8 x 8 x 1024 = 65,536 bits of memory whereas if it

is using 16-bit words, it now has 16 x 8 x 1024 = 131,072 bits of storage with the same

8K memory. So, it is important to know the word size of any given PLC before memory

size cab be accurately compared.

Table in Figure 27

RAM / ROM / EPROM / Firmware

Random Access Memory (RAM) is the most common type of volatile memory

used in all computers. Information can be written into, or read from, a RAM chip, and it

is often referred to as read/write memory. Random access refers to the ability of any

location or address in the memory to be accessed or used. Ram is used for both the user

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memory and storage memory in all PLCs. Since RAM is volatile, it must be battery

backup to retain or protect the stored program. RAM chips are manufactured with various

technology and CMOS-RAM is one of the most popular one. CMOS-RAM has very low

current drain when not being accessed (15µ amperes!). So a CMOS-RAM can be battery

backed up with a lithium batter which is rated 2.95 V at 1.75 ampere/hours and normally

can hold or protect a program for 60 days!

Read Only Memory (ROM) is a common type of nonvolatile memory and it

means that the information stored in memory can be read only, and cannot be changed.

PLC manufacturer places information in the ROM for internal use and operation of the

PLC, and it is not supposed to be changed or altered.

Erasable programmable Read Only memory (EPROM) is other type of

nonvolatile memory. An EPROM is ideally suited when program storage is to be semi

permanent, or additional security is needed to prevent unauthorized program changes.

The EPROM chip has a quartz window over a silicon material that contains the electronic

integrated circuit. This window is normally covered by an opaque material, but when it is

removed, and the circuitry exposed to ultraviolet light, the memory content can be erased

and then it can be reprogrammed with a special equipment called “EPROM

Programmer”.

Firmware is user – or application specific software programmed into special

memory contained in the hardware itself. In our case, it can be programmed into an

EPROM and delivered as part of the PLC hardware. It gives the PLC its basic

functionality.

Figure 28

Memory Structure

The memory of the most PLCs is divided into three areas:

1- Program space, data space, and configurable space.

2- Data space

3- Configurable space

Program space contains the ladder program instruction programmed by the user.

The instructions are entered either by a programming device, hand-held or desktop-type ,

or a system computer. This area of memory controls the way data space and I/O points

are used. LDA (Ladder logic) or STL (statement list) instructions are written using a

programming device and then loaded into this memory area.

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Data space is where the status (ON or OFF) of all input and output devices is

stored. Numeric values timers and counters (preset and accumulated), numeric values for

arithmetic instruction, and the status of internal relays also are stored in this area of

memory.

Configurable parameter space stores either the default or modified configuration

parameters.

Figure 29

Figure 29 show how the written ladder logic program after being change to some

binary codes is transferred into the PLC’s memory and is going to be executed by the

PLC at hand.

Minimum hardware and software Requirements to develop ladder logic control

programs

In order to modify or edit a program, you need the following.

1- PLC

2- Programming device

3- Programming software

4- Connector cable

A programming device (PG) is needed to enter, modify, and troubleshoot the

PLC programs, or to check the condition of the process. Once the program has been

entered and the PLC is running, the PG may be disconnected. It is not necessary for the

PG to be connected for the PLC to operate. A PG on the other hand can be used to

monitor the PLC program while the program is running. Programming devices come in

three types: hand-held, dedicated desktop, and computer.

Software

Many companies such as Siemens and Rockwell Software have developed

software for programming PLCs. A software program, running on a PC, may be used to

create a program for the PLC. This programming software is typically specific to one

PLC or family of PLCs. As an example, RSLogix ® software created by Rockwell

Software for programming the Allen-Bradley family of PLCs. This software in various

versions, can be used to program PLC-5, SLC 500, or the MicroLogix ® family of

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processors. It is Microsoft Window ® based and very user friendly. The S7-200 also uses

a windows-based program called step 7-micro/Win32 ®, which may be installed on a PC

in the same way as any computer software.

Cables

A special cable is needed when a personal computer is used as the programming

device to download the developed programs into the PLC’s memory. This cable, called a

PC/PPI cable, allows the serial interface of the PLC to communicate with the serial

interface of a PC. Some of these connecting cables have DIP switches on the PC/PPI to

select the appropriate speed (baud rate) for passing information between the PLC and

computer.

The S7-200 and Allen-Bradley MicroLogix 1500 Micro PLCs

Examples of basic programming techniques of typical PLCs are discussed and

illustrated in this section of this manual. For the sake of educational purposes, Many of

the examples used in the text are based on the Allen-Bradley MicroLogix as well as the

S7-200 micro PLC which the smallest member of the SIMATIC family of programmable

controllers . These two manufacturers are considered to have a large share of U.S.A and

Europe PLC market accordingly.

In both controllers, the CPUs are integral to the motherboards and inputs and outputs

connect them to the system being controlled. The inputs monitor field devices, such as

switches and sensors. Outputs control devices such as contactors, signal lamps or pumps.

The programming port is used to connect to the programming device.

Figure 30 illustrates Allen-Bradley MicroLogix 1500 ®

PLC with hard-wired I/O terminals. Courtesy of Allen Bradley

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Figure 31 illustrates the graphical picture of the Allen- Bradley

MicroLogix 1500 ® PLC

Figure 32 illustrates graphical SIMATIC

S7-200 PLC with hard-wired I/O terminals.

PLC Models

There model descriptions published by manufacturer of any brand of PLC which

gives the end user information about the CPU types, power supplies available to model.

Model description also indicates the type of inputs / outputs (if they are relay type or

transistor ones, AC or DC.). Other features related to each PLC such as amount of

memory, data backup time, number of I/O and expansion modules available for each

model can also be obtained from the said documents.

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Optional Cartridge

The S7-200 provides a super capacitor that maintains the integrity of the RAM

after power has been removed. Depending on the model of the S7-200, the super

capacitor can maintain the RAM for several days. The S7-200 also supports an optional

cartridge that extends the amount of time the RAM can be maintained after power has

been removed from the S7-200. The battery cartridge provides power only after the super

capacitor has been drained.

Expansion modules

S7-200 PLCs are expandable and various expansion modules are designed to give

these PLCs extra capacity to add additional digital/analog inputs, outputs and other

functions such as protecting over-voltage analog input modules.

Expansion modules are connected to the base unit with a ribbon connector.

The number of expansion modules that one can connect it to a PLC also depends on the

model of that particular PLC. For example for S7-200 CPU 224 the limit is maximum

number of 7 units and even then one needs to check the power budget to be sure he does

not overload the CPU power output. Check specification sheets when it comes to

expansion modules.

Understanding controller Status Indicators

The controller status LEDs provide a mechanism to determine current status of the

controller if a programming device is not present or connected. When yellow one is on, it

means CPU is in the STOP mode. When the mode is set t to RUN, the green RUN

indicator will be lit. Red led means system is in fault status. Activation of any terminals

of input or output will cause the related Green led to be turned on. Application of these

indicators is very useful when testing or debugging control logic ladder programs. See

figure 33.

Figure 33

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Using a DIN Rail to Install a PLC and Expansion units

PLCs usually can be installed inside of cabinets in one of two ways. The base

unit and expansion I/O DIN rail latches lock in the open position so that an entire system

can be easily attached to or removed from the DIN rail. Most PLCs also have holes on

panel which can be used to mount the PLC inside the any control cabinet.

Figure 34

External Power Supplies

Depending on the CPU model, Allen-Bradley model 1764-24BWA ® can be

connected only to 120/230 VAC power source. I/O numbering and power supply

terminals are all indicated on top and bottom terminal block layouts. Following figure

illustrates the way the A-B PLC is wired to 120 VAC supply. S7-200 PLCs can be

connected to either 24 VDC or 120/230 VAC. Power sources. On the other hand, S7-222

DC/DC/DC would be connected to 24 VDC. But S7-200 AC/DC/Relay model is

supposed to be connected to 120/230 VAC power source. See next two figures.

Figure 35

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Figure 36

I/O Numbering

The inputs and outputs are all identified by their addresses, the notation used

depending on the PLC manufacturer. These addresses are used by the CPU to determine

which inputs are present and which outputs need to be turned on or off. This is the

address of the input or output in the memory of the PLC. Siemens precedes input

numbers by I and outputs by Q. The first number in the designator identifies the byte, and

the second number the bit, of the I/O address. With the Siemens SIMATIC, the inputs

and outputs are arranged in groups of 8. Each 8 group is termed a byte and each input or

output with an 8 is term a bit Thus, Q2.0 means an output at bit 0 in byte 2. With larger

PLCs having several racks of input and output channels, the racks are numbered. With

the AB, the rack containing the processor is given the number 0 and the addresses of the

other racks are numbered 1, 2, 3, etc. according to how set-up switches are set. So, in

input address I:012/03 for example, “I” indicates an input, rack 01, module 2 and

terminal 03.

Figure 36a, Siemens and Allen Bradley addressing

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Inputs

Input devices, such switches, pushbuttons, and other sensor devices are connected

to the terminal strip under the bottom cover of the PLC. In figure 37, two pushbuttons,

proximity sensor and limit switch, all are considered as input devices.

Figure 37

Outputs

The output ports of a PLC are of the relay type or opto-isolator with transistor or

TRIAC types depending on the devices connected to them which are to switched on or off.

These output devices are connected to the terminal strip located under the top cover of

the S7-200 PLC. When testing a program, it is not necessary to connect output devices.

The LED status indicators “on” if an output is active. In the following figure, the red

signal lamp and the contactor to power motor are considered as output devices.

Figure 38

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Super Capacitor

The Super capacitor is an electrochemical energy storage applied in “power”

industries. Compared with battery, a super Capacitor has one-tenth of energy, but delivers

over 10 times power due to ultra low ESR (Equivalent Series Resistance). They also have

a much higher power density than batteries or fuel cells. These capacitors can provide

backup power for maintaining charge for a long period of time and protect data stored in

PLC’s RAM during power losses. The RAM is typically backed up for 50 hours on the

S7-221 and 222, and for 72 hours on the S7-224 and 226.

PLC Reference Manuals

A reference manual is a document often organized aphetically, designed as a

quick reference for experience users. A reference manual typically contains the most

frequently referenced subset of information including basic setup instructions,

troubleshooting for the most commonly encountered problems, and or prominent

features. Both Siemens and AB have lot’s of information in their websites regarding their

line of products.

www.ab.com

ab.rockwellautomation.com/Programmable-Controllers

www.siemens.com/

www.automation.siemens.com/_en/s7-200/index.htm

www.automation.siemens.com/.../simatic-s7.../s7-200/.../Default.aspx

PLC Display and HMI (human-machine interaction) Units

A user interface is a system by which people (users) interact with a machine. The

user interface includes hardware (physical) and software (logical) components. User

interfaces exist for various systems, and provide a means of:

1- Input, allowing the users to manipulate a system

2- Output, allowing the system to indicate the effects of the users manipulation.

Generally, the goal of human-machine interaction engineering is to produce a user

interface which makes it easy, efficient, and enjoyable to operate a machine in the way

which produces the desired result. This generally means that the operator needs to

provide minimal input to achieve the desired output, and also that the machine minimizes

undesired outputs to the human. Usually PLCs are designed such that could be used to

communicate with a variety of external devices such as HMI devices. HMI devices can

display messages read from the PLCs connected to and allow for adjustment of program

variables, provides forcing ability, and permits setting of time and date. The summarized

description of few HMI devices made by Rockwell Automation and Siemens is

mentioned here for your information.

Mfr. Part Number: 1760-DUB made by Rockwell Automation.

Description: Allen Bradley 1760-DUB multi-function Pico GFX-70 Display unit with

Keypad, displays text, date, and time, as well as custom bitmaps.

Mfr. Part number 20-HIM-C3 S/A made by Rockwell Automation

Allen Bradley 20-HIM-A3 PwerFlex Architecture Class Hand-Held Human Interface

Module, LCD Display, Full Numeric Keypad, Series C

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Allen Bradley 2711-B5A2 PanelView 550 monochrome Terminal 5.5 inch, Keypad &

touch screen, DH-485 communication Ports, AC power, Series F. combination for

convenient and flexible operator input: RS232 printer port, alarms, alarm lists, triggered

messages, and triggered states of a multi state indicator, field-replaceable backlights.

AB 1201-HJ2 programming terminal LCD Display

Siemens LOGO! Text display 6ED1 055-4MH00-0BA0

The new LOGO! TD text display panel provides an affordable HMI for equipment

builders and their customers, even on the simplest relay control systems. By having a

display panel with built-in operator functions and diagnostic messages customized for

their process, end users can now make quick adjustment or easy troubleshoot….

Siemens TD200

The Siemens TD 200 is the proven HMI device for the SIMATIC S7-200. In

addition to the display of alarm texts, it enables interventions in the control program (e.g.

set point value changes) or the setting of inputs and outputs. The TD 200 is suitable for

simple operation tasks with the SIMATIC S7-200 PLC. The focus is on the display of

alarm texts. The low total height and device depth make it the unit of choice even in

cramped space conditions.

Computer network

A computer network is a collection of hardware components and computers

interconnected by communication channels that allow sharing of resources and

information. The SIMATIC S7-200 micro PLC provides a full range of communication

capabilities. The integrated RS485 interfaces can be operated at data transmission rages

from 1.2 to 187.5 k. baud. A total of 31 units can be interconnected using a

communication cable.

PROFIBUS connection

All CPUs from 222 upwards can be run via the EM277 communications modules

as a norm slave on a PROFIBUS DP network with a transmission rate of up to 12 Mbit/s.

the open feature of the S7-200 to higher level PROFIBUS DP control levels ensures you

can integrate individual machines into your production line. With the EM277 expansion

module, you can implement PROFIBUS capability of individual machines equipped with

S7-200.

Powerful AS-Interface connection

The CP 243-2 turns all CPUs from 222 upwards into powerful masters on the AS-

interface network. According to the new AS-interface specification v 2.1, you can

connect up to 62 stations, making even analog sensors easy to integrate. With AS-

interface, you can connect up to 62 stations, making even analog sensors easy to

integrate. With AS-interface, you can connect up to 248 DIs + 186 DOs in the maximum

configuration. The max, number of 62 stations can include up to 31 analog modules. The

configuration of the slaves and reading/writing of data is supported by the handy AS-

interface Wizard.

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Figure 39

Programming a PLC with a Computer

Many companies have developed software for programming PLCs. For purposes

of illustrating how the software works, and the relative case of programming, we have

selected Step 7-Micro/Win ® created by Siemens for programming S7-200 family of

processors. The software is Windows® based and very user friendly.

With Step 7-Micro/Win software one can create any control program by arranging many

instructions that are arranged in a logical order.

Typical PLC Programming Instructions

Contact & coil Symbols (ladder format)

Generally, not all PLC manufacturers use the same notation and format when

labeling inputs, outputs, and the diagrams. Figure 40 illustrates a ladder logic diagram for

a simple circuit, where a single switch is controlling an output for two different

manufacturers of PLCs: (A) Allen Bradley and (B) Siemens.

Figure 40

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In Ladder Logic programming format, different electrical symbols are used to

write the program. For example, a contactor is consisted of a coil and some contacts

which are usually either normally open (NO) or normally close (NC).

The main function of the PLC program is to control outputs based on the condition of

inputs. The symbols used in ladder logic programming can be divided into two broad

categories: contacts (inputs) and coils (outputs). Most inputs to a PLC are simple devices

that are either on or off. These inputs are sensors and switches that are either “ON”

(pressed pushbutton) or “OFF” (inactive).

Coils are output symbols. Outputs can take various forms: motors, lights, pumps,

counters, timers, relays, and so on. A coil is simply an output or a “Load”. The PLC

examines the contacts in a ladder and turns the coils on or off depending on the condition

of the inputs. Figure 41 shows the two common symbols for contacts and a basic coil.

Figure 41

Figures 42 & 43 show a few possible inputs & output devices accordingly. A few

possible inputs including (from left to right), proximity switch, limit switch, floater

switch, limit switch, 3 different types of pushbuttons.

Figure 42

Next figure shows a few possible output devices. A few possible outputs including (from

left to right), contactor, motor, solenoid valve, signal light, pump, and pneumatic valve.

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Figure 43

Input/output & Contact programming examples

Following are 2 representative examples of PLC programming using contacts and coils.

For these examples, both PLC and relay logic solutions are shown.

EXAMPLE 1

The first example is a simple circuit with one toggle switch as a contact and one output as

a Lamp. As the switch is pressed or not, the output goes on or off (figure 44). Next two

figures show the relay logic (45B) and ladder logic (45C) diagrams.

Figure 44

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Figure 45

EXAMPLE 2

The second example is a start / stop seal-in circuit. When the start button is

pressed, the coil energizes. When the button is released, the coil remains on. It is held on

by a sealing contact that is in parallel with the start button. The seal contact closes when

the output coil goes on. If the stop button is pressed, the coil goes off and stays off. Also

if the control power goes off, the coil goes off too. The advantage of this example over

the first one is that when failed control power returns, stat pushbutton must be pressed to

reenergize the coil.

Figure 46 connection diagram

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Figure 47 Relay Logic diagram

Figure 48 Ladder Logic program

Functions mostly used in PLC programming

In this section we discuss some functions that are mostly used in programming

PLCs. And also short samples of written codes are represented for each function. These

functions are: AND, OR, Latch, Inverse ..etc.

AND function

In figure 49 let’s assume that we wish to activate output K1 any time 3 inputs

are activated in the same time. In this case we need to use AND function.

Figure 49 displays the Circuit and logic diagram for AND function. Ladder diagram of

AND function is also given. Anytime S1=S2=S3= 1 (all pushbuttons depressed) > K1 =

1 or High or on.

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Figure 49

OR function

In figure 50, K1 = 1 if any of inputs S1, S2 or S3 to be closed. Figure 51 displays Circuit

and logic diagram, and the LDA and STL programs respectively.

Figure 50

Figure 51 LDA and STL programs of OR function

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Latch Function

This function has lots of application. Let’s assume we wish to have a circuit

that anytime inputs S1 and S2 to be activated, K1 > ON and if S1 = 0 (OFF), K1 stays

ON (activated). K1 becomes un-activated if S2 = OFF (open). This function is used to

start electric motors in industry fields. In practice, a normally open switch is used instead

of S1, and a normally closed switch instead of S2.

In figure 44 notice that Q2.0 is used as parallel contact to I0.1 or in motor circuit, K1 is

used as a parallel contact to switch S1. Usually, most contactors or relays, have some

extra contacts that could be used with the same name (of the contactor) in the circuit

diagrams. As an example, K1 represent a contactor coil which is activated with say

120/240 volt AC. It has also an extra contact which is parallel to S1 switch (which also

labeled as K1).

Notice that only one bit of memory is allocated to each contact (or addresses such as

Q2.0) anytime as an example I0.1 = 0 means it is OFF or I0.1 = 1, it is ON. Since any

PLC usually has many banks of memory, therefore, a large number of contacts are

available to a software developer to use.

Figure 52 displays circuit and logic diagrams of Latch function.

Inverse function

We wish to have a circuit diagram to have its output K1 = 1 any time S1 = S2

= 1 but S3 = 0 (or OFF). Circuit Diagram of 53 displays such a circuit. Now, to simulate

S3 = 0 situation, we may use an Inverse function in our ladder logic program. Figure 53,

displays the ladder logic program for SIMATIC S7-200 PLC.

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Figure 53

The NOT gate is an electronic circuit that produces an inverted version of the

input at its output. It is also known as an inverter. If the input variable is A, the

inverted, output is known as NOT A. This is also shown as with a bar over the top.

In ladder logic programming, we may use a normally close contact, to simulate the

Inverse function. Figure 53 displays the how contact I0.3 represents inverse function of K

in the given LDA or STL software program.

FBD (Function Block Diagram) programming language

Function Block Diagram is a graphical programming language which enables the

user to rapidly program both Boolean and analogue expressions. The FBD editor offers

fast programming by placing boxes that each block does some type of logical function.

Figure54 illustrates a small program written in FBD format to function as an AND Logic

circuit.

Figure 54

STL (Statement List) programming language

In programming in STL language (or format), mnemonic codes are used, each

code corresponding to a ladder element. The codes used differ to some extent from

manufacturer to manufacturer. A program written in STL format, consists of different

statements grouped to form a list of instructions. Each instruction (statement) is usually

made of one logical block such as AND, OR, NOT … etc which is shown by an English

letter. English letter A, stands for logical block, AND: AN for NOT: O for OR and

finally”=” for OUT. A program can also consist of some other software devices such as

Flip Flops, Counters, Timers.. etc. As an example of STL programming, figure 55

displays a simple two inputs AND circuit.

Figure 55 shows the program for two input AND circuit

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Normally Open (NO) & Closed (NC) contacts

This contact gets activated anytime an input device such as a push button switch

is depressed. In this case, a “1” or “HGH”, or if the input is a DC type, 24 VDC appears

at the PLC’s input port channel. Obviously, if the contact gets de-activated, input voltage

level changes into “0” or “low“, or “ 0 VDC”. Figure56 illustrates, symbol used to

indicated a “NO” contact in ladder logic program. This contact functions apposite to what

was said about Normally Open contact. It means anytime, the push button is not

activated, input signal is considered as a “HIGH” or “on” or =1, and when it is activated,

input signal to PLC is “LOW” or “off” or = 0.

Figure 56

A PLC can only scan its input ports and register if any of them it is HIGH or

LOW , but it has no way to know if a NO or NC pushbutton is connected to any of its

input port. As an example, in figure 57, if S1 in CKT 1 is depressed, light bulb turns on

and turns off if it changes its state.

In the second circuit, (CKT2) again if we download USER PROG. 1 and execute it and

then depressed SP, light bulb goes on and turns off if pushbutton is de-activated.

BUT notice if we download the second USER PROG. 2 and execute it, as soon as PLC

starts to execute the program, light bulb turn on and stays on as long as pushbutton is not

touched otherwise, it turn off. So the conclusion is that with the same pushbutton, (an

input device), we get different result just by changing the software and this is beauty of

using a PLC in control processing circuits.

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Figure 57

Boolean Algebra PLC programming

We can program a PLC based on the Boolean algebra system, which is a

shorthand method of writing logic gate diagrams. Complex gate diagrams can be

analyzed easily when they are written in Boolean form. The analysis is covered in digital

logic texts. Here we cover only the PLC programming aspects of Boolean algebra.

The symbols used in the Boolean algebra system are illustrated in figure 58. Examples of

typical usage and the meaning of the Boolean expression in words are also given.

Figure 59 shows some typical gates and how they would be represented in Boolean form.

Figure 58

A conversion example

The example is about a motor control circuit with two start and stop pushbuttons.

When any of the two start button is depressed, the motor runs. By sealing, it continues

to run even when the start button is released. Depressing any of the two stop

pushbuttons, causes the motor to stop running. Figures 59, 60, 61 and 62, illustrate the

solution in four different formats: (59) Gate logic (60) Boolean expression. (61) Relay

logic (62) PLC logic (LDA)

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Figure 59

Figure 60

Figure 61

Figure 62

Example 1 is a start/stop seal- in circuit. When the start (I0.0) button is depressed,

the coil (LED in figure 1-48) turns on. When it is released, the coil remains on. It is held

on by a sealing contact that is in parallel with the start button. The seal contact closes

when the output coil goes on. If the stop button is depressed (I0.1), the coil goes off and

stays off. Also, if the control power goes off, the LED (coil) goes off. When failed power

returns, start button (I0.0) must be depressed to reenergize the coil. Figure 54 illustrates

the circuit in Gate Logic and LDA version of the given example1.

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Figure 63

Figure 64

As it was mentioned previously, the origin of PLCs are in the use of relays to

perform automatic functions and the wiring diagrams for relay logic circuits witch

resembled a ladder. In fact, PLCs replace the physical relays with imaginary ones that

are part of the written control program being executed within a PLC. PLCs input/output

terminals are physically connected to a set of input/output field devices such as sensors or

switches and control the on or off status of the output contacts. The control program

within the PLC determines the way the inputs control outputs. As it was illustrated in

figure 21, there are few methods of programming a PLC and in this book we will use

LDA and FBD format of programming.

Status Functions

After a program has been loaded and is running in the PLC, the status of ladder

elements can be monitored using the STEP 7-micro/WIN32 software. The standard

method of showing ladder elements is in the de-energized or non-operated state. When

viewing ladder diagrams in status mode, control elements that are active are highlighted.

Figure 65

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Figure 66

Force Function

Many PLCs have the capability to carry out a FORCE function. The function is

essentially an override control that is considered as another useful tool for program

debugging. When turned on, it can lead to feeding the process program with incorrect

information. It is used to temporarily override the input or output status of the

application during testing. It can be used to override discrete output points or to skip

portions of a program. The function must be used with utmost caution in conjunction with a

working process. Figure 67 shows in our test circuit, toggle switch is “off”, hence I0.0 =

off so Q0.0 = 0 or “off”.

Figure 67

Circuit in figure 68 shows the same circuit but this time by use of Force function, I0.0 is

turned “on” so the AND circuit consisting of I0.0 and I0.1 is set to 1 therefore, Q0.0 = 1

and the lamp which is wired to Q0.0 output is turned on or set to 1. As you noticed, in

figure 68 turning FORCE on, changed the statues of the contact (I0.0 = 1) on.

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Figure 68

You noticed that turning FORCE function on, changes the status of a contact or

coil. Meaning If it is a normally open contact, it will close (turn on), and if it is normally

closed contact, it will open (turn off). If you force a coil or function, it will go on when

forced. See figure 69.

Figure 69

Sequence

For the simple lamp circuit, the following is the sequence of events. Using its

program, the CPU scans the inputs. When it finds the switch open, I0.0 receives a binary

0. This instructs Q0.0 to send a binary 0 to the output module. The lamp remains off.

When the switch is closed, I0.0 receives a binary 1, which instructs Q0.0 to send a binary

1 to the output module. This turns the lamp on.

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Figure 70

Figure 71

3 Phase Motor Starter Diagram

Figure 72 shows the diagram of a motor start and stop circuit. The diagram shows

how a normally open and a normally closed pushbutton might be wired to start and stop

a 3 phase motor. The motor starter coil (CR) is wired in series with both pushbuttons.

The starter’s auxiliary contact, CR, is wired in parallel with the normally open

pushbutton. See figure 72.

Figure 72

When “START pushbutton “is pressed, current flows through STOP pushbutton ,

START pushbutton , CR coil of contactor , and overload protection circuit to “N” pole

of power supply. CR coil is activated, causing main contacts of CR and auxiliary contact

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of CR to close. See figure 73. Hence, when START pushbutton is released, motor is

going to continue being ON. See figure 74.

Figure 73

Figure 74 – STOP pushbutton is released

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Wiring Motor Starter circuit with PLC

Figure 75 shows the motor starter control task which can also be accomplished with a

PLC.

Figure 75

Writing a Ladder Logic Program for circuit for figure 75

Figure 76 shows the control ladder logic program written for circuit in figure 75.

For the sake of clarity, in figure 75, I replaced the good old START pushbutton with a

toggle switch you can see the status of the contacts inside of the switch better. Figure 76,

shows the status of PLC after the developed program is download into PLC memory and

RUN command is executed. Since, I0.1 and I0.2 both seen as NC contacts, programming

software displays them as turned ON field devices. As it was mentioned previously,

when PLC scans the inputs, Ladder Logic program tells the PLC that those two inputs

I0.1 and I0.2 are NC contactors and that is the PLC’s way to show those as turned “on”

elements. Since logically, neither of I0.0 nor Q0.0 is “on”, then current can not flow

through none of those devices to activate the main contactor (CR). That is why Q0.0 = 0

or off so the 3 phase motor stays “off”. Figure 77 shows the circuit status when START

toggle switch is activated.

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Figure 76

Figure 77 shows the circuit status when START toggle switch is activated.

Figure 77

As explained circuit in figure 77, when toggle switch is pressed, current flow through

AND circuit consisting I0.1, I0.0 and I0.2. All these three inputs are used in AND

configuration to control Q0.0. Also, a normally open set of contacts associated with Q0.0

is programmed to form an OR circuit. Output Q0.0 is used to control the motor starter.

Expanding the previous problem

Now that we have mastered writing Ladder Logic programming, let’s do another

one with a little more number of inputs and outputs devices wired to our simple PLC.

There is a mach table which has three pushbutton switches S1, S2 and S3. Design circuit

diagram and control program such that if these switches be hit at the same time, only the

one which is hit first, to be turned on. Figure 78 shows the relay logic of the problem.

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Figure 79 the related Ladder Logic program, and finally figure 80 the connection of field

devices to the PLC.

Figure 78

Figure 79

Figure 80

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Introduction to Analog signals

An analog is any continuous signal for which the time varying feature (variable)

of the signal is a representation of some other time varying quantity. It differs from a

digital signal in terms of small fluctuations in the signal which are meaningful.

Electrically, the property most commonly used is voltage followed closely by frequency,

current, and charge. Temperature is the most measured process variable in industrial

automation. The two prime sensors used to measure it are resistance temperature

detectors (RTD) and thermocouples (T/Cs).

RTDs are usually remotely located (they are not located directly next to the PLC).

So, wires have to be used to connect them to the monitors or PLCs. Hence, when we need

to connect a RTD to a PLC, then PLCs must be able to work with continuous, or analog,

signals as well as discrete. Typical analog signals range from 0-10 or 0-5 VDC or 4-20

mA. From our previous examples, you noticed that all our input devices were discrete,

meaning their produced output voltage was either 0 (off) or 1 (on). Since a PLC can not

process signals in analog form. Therefore, Analog signals must first be converted to

digital representation, which is accomplished by an expansion module. Both Siemens and

Allen Bradley have designed Expansion analog isolated input and output modules. By

connecting these modules to the related PLCs, user can connect verities of sensor our

output devices which their application requires analog signals. These analog modules

convert standard voltage and current values to 12-bit digital representation. These digital

values are transferred to the PLC for use in its program. Analog modules are also

available for use with thermocouple and RTD sensors for accurate temperature

measurements.

Analog modules made by Allen Bradley for MicroLogix family of PLCs

1769-IF4I isolated analog input module

1769- OF4CI isolated analog output module

1769-OF4VI isolated analog output module

Analog modules made by Siemens for S7-200 family of PLCs

EM 231 RTD SIMATIC S7-200 ANALOG INPUT MOD (6ES7 231 – 7PB20- 0XA0)

EM 235 SIMATIC S7-200 ANALOG I/O (6ES7235 – 0KD20-0XA0)

Application Example

Figure 81 shows that a typical expansion module is connected to a PLC via a flat

cable. RTD is a field device that can be used to measure a varying value that in our case

is “current”. Inside circuitry of the expansion device is such that it applies small amount

of voltage to RTD and measures the related current. It then based on the measurement,

calculates the temperature value. Based on the control ladder logic program, PLC make

to decision what to do based on the calculated temperature value.

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Figure 81

Analog Output signals

Analog applications are present in many forms. Figure 82 shows the controller

that controls the amount of fluid in a holding tank by adjusting the valve opening. The

valve functions based on the analog input signal received from the PLC. The valve is

initially open 100% in this case maximum amount of fluid can flow into the tank. As the

fluid level approaches the preset point, the controller modifies the output. Controller

reduces the amount of flow of fluid little by little continuously adjusting the valve to

maintain the fluid level and finally shuts it off. Analog outputs can be provided by

digital-to-analogue converters at the output channel. The input to converter is a sequence

of bits with each bit along a parallel line. Next figure shows the basic function of the

converter. By using a converter module (a suitable expansion module) we can change 8

or 16 bits of PLC output to an analog output module. Typical analog signals range from

0-10 or 0-5 VDC or 4-20 mA. Figure 71 shows that analog l output signal of a PLC is

wired to a valve to control the flow of fluid.

Figure 82

Could you explain how does the process in figure 82 work?

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Introduction to A-B MicroLogix and SIMATIC S7-200 Timers

In many control tasks there is a need to control time. For example a motor might

need to be controlled to operate for a particular interval of time, or perhaps be switched

on after some time interval. PLCs thus have timers as built-in devices. Timers count

fractions of seconds or seconds using the internal CPU clock.

Hard-Wired Time Delay relay

A Time Delay relay is a combination of an electromechanical output relay and a

control circuit. The control circuit is comprised of solid state components and timing

circuits that control operation of the relay and timing range. Most common typical time

delay functions include On-Delay, Off Delay, Retrigger able, One Shot Timer and few

other types.

Prior to invention of PLCs, different types of electromechanical timers were only

available to many control tasks which there was a need to control time. For example, a

Stop light control system used to be built with tens of hard-wired electromechanical

timers.

Timers used with PLCs can be compared to hard-wired timing circuits. In the

following example, we first examine how a Time Delay Relay functions, and then discuss

timers used with PLCs. See figure 83.

Figure 83

According to the timing chart (On Delay timer), when switch is pressed, input

voltage I (enable signal (L1) is applied to timer TR1, timing delay t1 begins (next figure

b). Relay contacts T changes state after time delay is complete (let’s assume

predetermined value for t1 = t2 = 10 seconds). Terminal # 1 connects to terminal # 3 and

lamp turns on (120 VAC is applied to the Lamp). Contacts T return to their shelf state

when input voltage T is removed. Lamp turns off. When I is applied again, after t2= 10

seconds, Lamp turns on and stays on as long as switch is pressed. Lamp turns off as soon

as switch is released.

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Figure 84 shows status of a, un-activated, and b activated timer

A-B MicroLogix 1500 and SIMATIC S7-200 Timers

Generally speaking, 3 most used types of timers may be discussed here. These are:

1- TIMER ON-DELAY (TON)

2- TIMER OFF-DELAY (TOF)

3- RETENTIVE ON-Delay Timer (RTO)

Timers are represented by boxes in ladder logic. When a timer receives an enable signal,

it starts timing, and continuously compares its current time with a preset time. The

timer’s output is logic 0 as long as the current time is less than the preset time. When the

current time exceeds the preset time, the output changes to logic 1.

A-B Timer On- Delay (ladder logic symbol and function)

Figure 85 displays symbol used in ladder logic

programs to represent On-Delay timer

An on-delay timer will wait for a set amount of time (preset value e.g. 20 seconds

in above figure) after a line of ladder logic has been true (Timer on Delay input) before

turning its output on (EN), and it will stay on as long as the line of ladder logic stays at

its true status (as long as Timer On Delay = 1).

Figure 85 shows that the timer consists of a timing block containing TIMER address

T4:0, TIM BASE which is set to 1.0 second, the PRESET value which is 20 seconds

and finally ACCUM which is = 0.

T4:0 is the timer format in which.

T Identifies this as a timer file

4 This is timer file 4 (default file)

0 This means timer 0 in file 4 which can be any number from 0 to 255.

In this case, there are 256 timers available in file 4 for the programmer to use

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An On-Delay Timer programming Example

Develop a program that will turn on light O0 for 2 second and off for 5 second.

Figure 86 ladder logic program developed Using an On- Delay timer

TIMER ON-DELAY (TON) Application

Timer on-delay (TON) is used any time we need to start some action after a

certain amount of time passed from the time an input signal is received.

As an example, let’s assume stop light turns from red to green and the driver drives the

car along the road. If he drives 35 mile / hour, (which is actable speed), it takes him only

10 minutes to get to the second stop light. Since he was a good boy driving within the

speed limit, he deserves to receive a green light at the next stop light.

The 10 minutes delay is the on-delay timer’s preset value.

A-B Timer Off-Delay (ladder logic symbol and function)

Figure 87

In winter, you have noticed that any time electric heater is turned off (by the house

thermostat), the blower fan motor dos not turn off right away. Instead, it stays on for

about 5 more minutes after the motor is turned off already and then it is turned down

automatically by the external circuit.

This is a five-minute off delay timer. The 5 minute timing cycle begins when the

blower motor is turned off.

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Above figure displays an Allen-Bradley TIMER OFF-DELAY (TOF) instruction symbol.

The timer consists of a timing block containing TIMER address T4:0, TIME BASE

which is set to 1.0 second, the PRESET value which is 20 seconds and finally ACCUM

which is = 0.

TIMER Off-Delay (TOF) Example

Develop a program that will turn on when Start button I0 is pushed and can be stopped by

I1. When O0 goes on initially the TON timer is used to sound the horn (O1) for the first

5 seconds to warn that the oven will start, and then the horn (O1) stops and the heating

coil (O2) starts. When the oven is turned off (by pressing I1), fan continues to blow for

10 seconds.

Figure 88 ladder logic program developed Using TON & TOF timers

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AB Retentive On- Delay Timer (ladder logic symbol and function)

Figure 89

A-B Retentive On Delay Timer functions similar to TON, except that it is

retentive. The most significant point about this timer is that when the input I0, turned off,

the accumulator value does not reset to zero. And since the ACC value does not reset to

initial value. A reset instruction can be used to reset the content of the accumulator to

zero.

The EN, TT and DN bits function the same as with the TON instruction. When rung 0

goes true, and stays true, true, timer continues to time until the ACC value equals to

preset value. In this case, DN >1, EN > 1 and TT > 0 (timing stops).

And the transition of input from 1 > 0, causes EN > 0 TT > 0 and DN > 1.

And in this case, bit 13 (DN bit) is set to 1 by the processor and remains ON as long as

the accumulated value is equal to the preset value.

Notice that RES instruction must be given the same address as the retentive timer used in

the rung we intend to rest it.

Figure 90 ladder logic program developed Using RTO timers

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SIEMENS SIMATIC S7-200 TIMERS

The Siemens S7-200 PLC uses also three types of timers: On-Delay (TON),

Retentive On-Delay (TONR), and Off-Delay (TOF). They are provided with resolutions

of 1 millisecond, 10 milliseconds, and 100 milliseconds, making the maximum counts

32.767 seconds, and 327.67 seconds, and 3276.7 respectively.

The total number of these Timers is 256 with different resolution of 1 ms (4 timers), 10

ms (16 Timers), and 100 ms (236 Timers).

Figure 91

According to Timer table illustrated in above in figure 91, should we need a TON timer

with 10 ms resolution, we can use Timers with numbers T33 to T36, or T97 to T100.

S7-200 On-Delay Timer (ladder logic symbol and function)

Figure 92 illustrates ladder logic symbol for On-Delay timer

According to the above figure, S7-200 timers are controlled with a single enabling input

and have a current value that maintains the elapsed time from the time that the timer was

enabled. The timers also have a preset time value (PT) that is compared to the current

value. A timer bit is set/reset based upon the result of this comparison. When the current

value is greater than or equal to the preset time value, the timer bit (T-bit) is turned on.

Otherwise, the T-bit is turned off. Timing stops when the current value reaches a

maximum value. When a timer is reset, its current value is set to zero, and its T-bit is

turned off.

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On-Delay Application Example

Write a program that when S1 is activated, after elapse of 5 seconds, output

Q0.0 = 1. S1 is a simple on-off switch.

According to Timer table, we wish to choose T33, TON timer with resolution of 100 ms.

How to calculate PT?

To calculate PT value, we must know our Timer’s resolution and the time delay after

which we want the Timer output to be activated. We need to calculate PT for a timer

based on condition that its resolution is 100 ms and timer time is 5 S.

Figure 93

S7-200 Off-Delay Timer (ladder logic symbol and function)

Figure 94 illustrates ladder logic symbol for Off-Delay timer

According to the above figure, when the enabling input (I0.0) turns on, the timer

bit turns on immediately, and the current value is set to 0. When I0.0 turns OFF, the timer

counts until the elapsed time reaches the preset time (PT). When the preset value =

current value, the timer bit turns OFF and the current value stops counting.

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In the following example, T33 with 1 ms resolution is chosen. Hence PT is calculated to

PT = 5000.

Figure 95

S7-200 Retentive On- Delay Timer (ladder logic symbol and function)

Figure 96

TONR timer functions similar to TON timer except in the cases when an On-

Delay timer current value is cleared when the enabling input is OFF, while the current

value of the Retentive On-Delay Timer is maintained when the input is OFF. Hence one

can use a TONR timer to accumulate time for multiple periods of the input ON. A Reset

instruction “R” is used to clear the current value of the TONR timer.

Exercise 1

Write a program that when S1 is activated, after elapse of 5 seconds, output of the TONR

timer Q0.0 = 1. I0.7 is a simple on-off switch.

According to Timer Table, T0 is a Retentive on delay timer with resolution = 1 ms.

Solution

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Figure 97

Counters

A counter allows a number of occurrences of input signals to be counted. This

might be in a situation where items pass along a conveyor belt and a specified number

have to be diverted into a box. It might be counting the number of revolutions of a shaft,

or perhaps the number of people passing through a door.

A-B SLC 500 and Siemens S7-200 COUNTERS

Depending on the Counter application, we have two types of counters designed to

serve the same function as the mechanical counters.

Generally speaking, 2 types of counters are:

1- Up-counter (CTU)

2- Down-counter (CTD)

AB counters are programmed almost exactly like AB timers discussed previously.

There is counter number, a preset, and an accumulated value. The counters are numbered

similar to timers except it begins with a “C” instead of “T”. Allen – Bradley SLC 500

and MicroLogix counter addressing is outlined as follows:

C5:3

C identifies the instruction as a counter file

5 the default file number (any unused file number from 10 to 255 can be assigned).

MicroLogix is limited to one counter file, which is default file 5. Hence, it is limited to

40 counters (0 to 39). SLC 500 can use files 0 to 255.

:3 “:” and number 3. Colon separates file number (5) from counter number which is # 3

this can be any number. There are 256 counters in each file number. In our case, C5:3 is

the forth counter from counter file 5. Each counter is an element and like timer, each

counter element is consists of 3 words:

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AB Up-counter (CTU) symbol

1- Up-counter (CTU)

Figure 98

Count Up Done Bit (DN)

DN bit is set to 1 and stays on when accumulated value is equal or greater than preset

value.

Count up Enable Bit (CU) Activating input I0, causes a positive edge signal to the counter which (rung 0 becomes

true) causes CU status bit to be ON and remains true as long as counter rung is true (rung

0). Clearly, CU is false when the counter rung is false or I1 (reset input) is activated.

In next figure, any time should input device I0 (false to true transition) to be activated,

causes ACC to clear and DN to be set to off.

Count up sample example

Develop a ladder logic program that will turn on a light after switch I0 has been closed

10 times. Switch I1 will be used to reset the counter.

Figure 99

2- Down-counter (CTD)

Count Down Enable Bit (CD)

CD >1 as long as counter rung is true. Hence with positive going edge of I0 (false

to true transition), notice that CD is also set to 1 and stays ON as long as the counter rung

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is true. Leading edge of I0 sets CD > 0 or false. Also notice that anytime rest instruction

is executed by activating input device I1 (0 to 1 transition), ACC> 0.

Count Down Done Bit (DN) > 1 or true as long as Accumulated value > or = Preset

value. Count Down Underflow Bit (UN) >1 any time ACC exceeds lower limit of < -

32,768. In this case, with the next activation of I0, UN is set to 1 and the next value of

ACC is going to be 37,767.

Count Down Enable Bit (CD)

CD > 1 as long as counter rung is true. Hence with positive going edge of I0

(false to true transition), notice that CD is also set to 1 and stays ON as long as the

counter rung is true. Leading edge of I0 sets CD > 0 or false. Also notice that anytime

rest instruction is executed by activating a reset input, (0 to 1 transition), ACC> 0.

Figure 100 an A-B CTD sample program example

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Count up application example

Develop the ladder logic that will turn on a light after switch I0 has been closed 10 times.

Switch I1 will be used to reset the counter.

Figure 101 illustrates a program developed to show an application of CTU counter

SIEMENS SIMATIC S7-200 COUNTERS

In generally speaking, there are two types of counters:

1- Regular Counters

A - CTUD ~ Count Up/Down counter

B – CTU ~ Count Up counter

C – CTD ~ Count Down counter

2- High-Speed Counters

A – PLS ~

B – HDEF ~

C – HSC ~ High-Speed Counter

High-Speed Counters are not supported by all S7-CPUs. And because discussion about

High-Speed counters and their application is beyond the scope of this guide, it will not be

discussed.

There are 256 counters in the S7-200, numbered C0 through C255. The same number

cannot be assigned to more than one counter. For example, if an up counter is assigned

number 45, a down counter cannot also be assigned number 45. the maximum count

value is +/- 32767.

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CTU ~ Count Up Counter

CU ~ Count Up input

R ~ Reset input

PV ~ Preset Value

Figure 102

Sample problem

In next figure, when CTU instruction receives 12 pulses on the rising edges of the CU

input (I0.0), C0 (counter bit) is activated and thus, Q0.0 = 1.

Figure103

CTD ~ Count Down Counter

CD ~ Count Down (CD) input

LD ~ load input

PV ~ preset value

Figure 104

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Sample problem

In the next sample problem, CTD instruction counts down from the preset value

(PV = 12) on the each rising edges of CD input till it equals to 0. And when current value

equals to = 0, C0 (current bit) turns on and Q0.0 = 1. LD input is to reset the Counter.

Figure 105

CTUD ~ Count UP/Down Counter

CU ~ count up input

CD ~ count Down input

R ~ Reset input

PV ~ Preset value

Figure 106

Sample Problem

In the next sample problem, Count Up/Down instruction counts up on rising

edges of the Count Up (CU) input (I0.0). It counts down on the rising edges of the Count

Down (CD) input (I0.2). When the current value (C0) is greater than or equal to the

Preset Value (PV= +6), the counter bit (C0) turns on. The counter is reset when the

Reset (R) input turns on (I0.0 is activated).

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Figure107

Interrupt function

An interrupt is a signal that causes the PLC to stop the current program forcing it

to execute another program immediately regardless of where the scan currently is .

Meaning the PLC immediately stops what its doing and executes an interrupt routine.

After it is done executing the interrupt routine, it goes back to point where it left off and

continues on with the normal scan process. Hence, Interrupts are another example of

instructions that must be executed before the PLC completes a scan cycle.

Pulse train output (PTO) function

Many applications require a series of pulses produced at specific intervals (a pulse

train). For some applications, a pulse train of a specific frequency is sufficient, but other

applications may require the pulses to be generated from an input signal, have a specific

duty cycle (the percentage of the pulse period that the pulse is high), or to start in

response to a particular input. Most PLCs manufactured today, have the ability to

produce pulse train to control an open / close loop stepper motor system. In the open loop

system, a clockwise signal or counter-clockwise signal is sent from the interface modules

to the stepper motor drive amplifier. This amplifier translates the single pulse train signal

sent from the interface module to the required number of signals for the stator windings

of the motor. when applied to the stator windings, the pulses from drive amplifier are

converted into discrete rotor movements, or steps. In the open loop system , a clockwise

or counter-clockwise signal which is determined by the PLC program , is sent from the

interface module to the stepper motor drive amplifier. See 108 in which output port Q0.0

is generating pluses to control a stepper motor.

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Figure 108

Pulse Width Modulation (PWM) function

Pulse-Width Modulation is a highly efficient an convenient way of controlling

output voltage to devices with large time constant, such as controlling the speed of a DC

motor, the power to a heating element or the position of a proportional valve. PWM

works by first turning the output to full voltage for a short while and then shutting it off

for another short while and then turn it on again and so on in accurate time intervals.

PWM function statement controls the frequency a duty-cycle of the PWM channel. In this

case, the PWM output channel can be used to directly control the speed of a small DC

motor or up to maximum of 1A current. They can also directly drive proportional valves

whose opening is dependent on the applied voltage. The advantage of using PWM is that

it can be easily amplify the drive current to a larger load such as larger permanent magnet

DC motor by using low cost DC Solid-State Relays (SSR) to boost the current witching

capability. In such case the PWM output is only used to drive the emitter portion of the

SSR. The load is driven by the receiver portion of the SSR. Using SSR has the added

advantage of isolating the CPU from the high current load.

Figure 109

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In figure 109, top shows the average voltage applied to the Load with the shown duty

cycle. In the bottom, it shows how the output Q0.1 PWM voltage is amplified and applied

to the DC motor. Figure110 illustrates the equations to calculate the Average voltage,

duty cycle, period, and frequency.

Figure 110

Voltage of 110 VAC @ 60 HZ is with duty cycle of 20% is applied to a resistive load.

Calculate the average voltage and period of the voltage applied to the load.

Average voltage is = (20/100) 110 = 2/10 x 110 = 22 Volt

F = 1/Period 60 cycle /Second = 1/period

Period = 1/60 second = 1000 ms/60 = ~ 16.66 ms

Transmit function

The Transmit blocks send Modbus messages from a "master" PLC to multiple

slave PLCs or sends ASCII character strings from the PLC’s Modbus slave port to

ASCII printers and terminals. The function can send these messages over telephone

dialup modems, radio modems, or simply direct connections. The Transmit blocks

perform general ASCII input functions in the communication mode including simple

ASCII and terminated ASCII. You may import and export ASCII or binary data into your

PLC. The block has built in diagnostics that checks to make sure no other Transmit

blocks are active in the PLC on the same port. Within the Transmit blocks, control inputs

allows you to control the communications link between the PLC and DCE (Data

Communication Equipment) devices attached to Modbus ports of the PLC. The Transmit

blocks do

NOT activate the port LED when transmitting data. Transmit allows communication with

external devices, such as modems, printers, and computers, via the serial interface.

Modbus is a serial communications protocol published by Modicon in 1979 for use with

its PLCs. Simple and robust, it has since become one of the de facto standard

communications protocols in the industry, and it is now amongst the most commonly

available means of connecting industrial electronic devices.

http://www.alamedaelectric.com/Modicon

http://kernow.curtin.edu.au/www/plc/3STANDAR.HTM

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Introduction to Network Communications

A local area network (LAN) is a computer network that interconnects computers

in a limited area such as a home, school, computer laboratory, or office building.

PROFIBUS (Process Field Bus) is a standard for field bus communication in automation

technology and was first promoted in 1989 by German department of education and

research and then used by Siemens.

PROFIBUS DP (Decentralized Peripherals) is a protocol made to operate sensors and

actuators via a centralized controller in productions (factory) automation applications.

PROFIBUS-DP allows the features of S7-200 PLCs to be used to their full extent within

a distributed system. Another advantage is that it allows uniform communication between

all SIEMENS SIMATIC devices on the PROFIBUS-DP network and with PROFIBUS-

DP ® devices from other manufacturers.

The PROFIBUS-DP EM 277 module allows the S7-200 CPU to be connected to the

PROFIBUS-DP network as a slave. The CPU 243-2 communication processor allows

communication between AS-I devices and S7-200 PLCs.

Figure 111

AS-Interface (AS-i) the Actuator Sensor Interface is not an universal Fieldbus

for all areas of automation, but rather an economically reasonable system for the lower

field level. The AS-i is a two-core cable specially designed to connect AS-I devices. The

yellow AS-I cable supplies 30V DC power to sensors and transfers data. The black AS-I

cable can be used for 24 V DC actuators. The Actuator-Sensor Interface is a digital,

serial, bi-directional communications protocol and bus system that interconnects simple

binary on-off devices such as actuators, sensors, and discrete devices in the field. Two-

conductor AS-I bus cable supplies both power and data for the field devices of up to

100m (more if extenders or repeaters are used). No terminators are needed anywhere on

the AS-I bus. Figure112 illustrates the cross section of a two-core AS-I cable.

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Figure 112

SIMATIC S7-200 from CPU 222 upwards has functionality as AS- interface master via

AS-interface module (AS-Interface master CP 243-2 max. 2 modules). Figure 113

illustrates an AS-I network of S7-200 PLC participating as a master module.

Figure 113

A thermistor is a type of resistor whose resistance varies significantly with

temperature, more so than in standard resistors. Thermistors are widely used as inrush

current limiters, temperature sensors, self-resetting over current protectors, and self-

regulating heating elements. Thermistors differ from resistance temperature detectors

(RTD) in that the material used in a thermistor is generally a ceramic or polymer, while

RTDs use pure metals. The temperature response is also different. RTDs are usfeful over

larger temperature ranges, while thermistors typically achieve a higher precision within a

limited temperature range typically -90 ºC to 130 ºC

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Definition of terms

What is a Sensor?

A Sensor is a device, which responds to an input quantity by generating a

functionally related output usually in the form of an electrical or optical signal.

During the past two decades, there has been an unprecedented growth in the number of

products and services, which utilize information gained by monitoring and measuring

using different types of sensors. The development of sensors to meet the need is referred

to as sensor technology and is applicable in a very broad domain including the

environment, medicine, commerce and industry.

What is an actuator?

An actuator is a transducer that accepts a signal and converts it to a physical

action. In other words, an actuator causes an action to occur relating to the data that was

sent to it. Data written to an actuator is held in an input register until the trigger is

received.

What is a TEDS?

A Transducer Electronic Data Sheet is known as a TEDS. It is a set of electronic

data in a standardized format stored in a chip that is attached to a transducer, therefore

allowing the transducer to identify and describe itself to the network, thereby easing

automatic system configuration. This self-identification capability for the transducer is

needed for maintenance, diagnostics, and to determine mean-time between failure

characteristics. The chip stores information such as manufacturer name, identification

number, type of device, serial number, as well as calibration data. The TEDS can be

uploaded to the system upon power up or request. It also serves as documentation for the

transducer.

What is a transducer?

A transducer basically converts a kind of energy or some kind of incoming energy

into some kind of electrical signal. Now the energy might be something like a movement

or sound or light, those are the kinds of energies we're talking about. Obviously the most

common ones which are used nowadays are the light sensitive components like photo

diodes, photocells. These directly convert the incident light into an electrical signal

which can be used to well for example detect how bright light is. A photo diode in a

camera detects how bright it is so that the camera's automatically set. You also have

sound sensing devices, these sound devices are often used again in for example an

automatic camera, an automatic recorder so that as the sound gets louder, it reduces the

amplification so that it doesn't make the sound particularly difficult to listen to.

Movement sensors obviously with many experiments in laboratories we like to detect

movement and movement can be detected either physically by using a sensor and

physically measuring the distance, it could be a distance sensor or it could be something

sensing that there is a movement and give you a signal accordingly.

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Opto-isolator

Also called an optocoupler, photocoupler or optical isolator, is “an electronic

device designed to transfer electrical signals by utilizing light waves to provide coupling

with electrical isolation between its input and output. Figure 114 displays an input

module in which two opto-isolators are used in its structure.Usually input signals are 24

VAC or VDC. Any of these signals go through a current limiter resistor, a LED and

finally connects to input of an Optocoupler (such as IC 4N25 made by VISHAY).

Figure 114 displays an application of an opt-isolator IC used in an Input Module

What is a TRIAC?

The TRIAC is a three terminal semiconductor device for controlling current. It is

effectively a development of the SCR or thyristor, but unlike the thyristor which is only

able to conduct in one direction, the TRIAC is a bidirectional device. Figure 115 displays

the TRIAC symbol. The symbol recognizes the way in which the TRIAC operates. Seen

from the outside it may be viewed as two back to back thyristors. This is what the TRIAC

symbol indicates.

Figure 115

What is a LED?

A light-emitting diode (LED) is a semiconductor light source. LEDs are used as

indicator lamps in many devices and are increasingly used for other lighting. LEDs, are

real unsung heroes in the electronics world. They do dozens of different jobs and are

found in all kinds of devices such as numbers on digital clocks, transmit information

from remote controls, light up watches and tell you when your appliances are turned on.

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Collected together, they can form images on a jumbo television screen or illuminate a

traffic light.

Figure 116 displays

(A) 3 x 5mm diameter LEDs

(B) 3 x 1.5 mm diameter Micro / Mini

LEDs ~ Red, Green and Yellow colors

PLC Mini-Tutorial

Introduction

This document is designed to give an overview of what a PLC is, what it is good for, what it is not best at,

an overview of its operation and where to find more information. A Programmable Logic Controller (PLC)

is a rugged special purpose computer that reads a bunch of input signals; runs control logic, and then writes

output signals. After reviewing this mini-tutorial please feel free to ask us questions or ask about a free

seminar and demonstration at your site.

Pros

1. PLCs are good at turning outputs on or off based on the state of inputs. (control) 2. PLCs are good at bringing together and concentrating a lot of data and status that is uploaded into

a computer in a compact form 3. PLCs are more rugged than computers and typically last five, seven, ten years without needing

replacement

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Cons

1. PLCs are not the best at handling large amounts of data or complex data. 2. PLCs are not the best at reading and writing databases. 3. PLCs are not the best at outputting resultant data to printers. 4. PLCs are not the best at displaying data and information to the operator.

History

Programmable Logic Controllers (PLCs) have been around since the dinosaurs. In fact it was PLCs that

killed the dinosaurs. Don’t listen to those people that tell you that dinosaurs were killed off by global

warming / cooling, meteors, or even smoking cigarettes. PLCs killed the dinosaurs. You see, millions of

years ago the cavemen used PLCs for everything they did. All of the dinosaurs lost their jobs, couldn’t

afford to buy food at the local stoney-mart and just died off.

Okay, okay I guess I have to admit that the last paragraph is not true. But the PLC has been around since at

least the 1960s, which in technology years makes it a dinosaur. For the real history of the PLC follow this

link http://www.barn.org/FILES/historyofplc.html/ to the website of Dick Morley, the inventor of the PLC

(just don’t forget where to come back to).

PLC Example

Let’s start off with a simple example. Suppose you are making a controller for an overhead crane. The

operator has a simple control box with four push buttons: "left", "right", "up", and "down". When the

operator presses the "left" button (note that the "left" button is an input to the PLC), then the PLC turns on

the appropriate output to the motor that makes the crane move left. The other three buttons would operate

similarly. Sounds pretty simple – right?

Suppose it takes ten minutes for the crane to reach the full left position. Soon the operator’s fingers start to

hurt (holding that button down for ten minutes at a time hurts), and they are going to beg / bribe / threaten

you, the programmer, to latch that output on and add a stop button. Instead of having to press the "left"

button for ten minutes, the operator wants to momentarily press the "left" button and the crane keeps

moving left till the operator presses the "stop" button. So you reprogram the crane and now the operator

picks up a 10 ton container, presses the "left" button, realizes he forgot to get a drink (of water), and

knowing that the crane will be moving for ten minutes, goes off to get a drink. Or suppose the crane hits the

operator and knocks them out. Who is going to stop the crane? There are some major safety considerations

since you have a 10-ton container moving around with no one to stop it.

So you start adding safety light curtains and mats around the crane’s operational area, so that if anything

comes into the crane’s operational area the crane automatically stops. You would also add Emergency Stop

(E-Stop) buttons around the area so that anyone can press one of these buttons to stop the crane. You would

want to add end-of-travel limit switches so that when the crane moved as far as it can go then the PLC

would automatically stop the motor. You would also want to add some more inputs (feedback) to the PLC

so that when a motor fault occurred the PLC would detect the fault, turn off the motor, and sound alarms.

There are many other safety and diagnostic inputs you should add.

Do you see how a very simple application can grow in inputs and outputs very quickly? The good news is

that by using a PLC for this application the PLC is very quickly and easily reprogrammed for the new

inputs. Other wise you have to go get more relays and do a bunch of wiring for each new input and output.

Even More Complexity

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We can extrapolate this simple crane into more complex systems:

A "crane" that automatically loads or unloads 55 gallon drums onto pallets, containers on or off a

ship, or adds a finite amount of reagent to a matrix of test tubes. Multiple cranes that have overlapping work envelopes and require collision avoidance and

cooperative handling "Cranes" that work in three-dimensional space to store and retrieve items. Applications from

electronics to pharmaceuticals show that automated storage and retrieval systems reduce errors

significantly. Two axis controllers that move a video camera around for inspecting parts

The control systems engineer sees a lot of similarities in these different applications. All of these

applications can use a PLC but these applications are just a tiny subset of all the control schemes that

employ PLCs.

Basic Signal

There are a lot of different types of signals that PLCs can read and write. The two most basic signals are discrete (digital) and analog. Discrete means on or off, 1 or zero, high or low, etc. Two possible states. Using discrete signals you could have a switch that when pressed starts a motor running. Both the switch “input” and the motor start “output” would be discrete signals. Analog signals are continuous signals varying between two limits. Analog signals can have a multitude of different values between those two limits such as pressure, temperature, level, etc.

Early PLCs were designed primarily as relay replacements. Those PLCs were designed completely around discrete applications. They were a fantastic improvement over the rooms of relays that were used to operate what would be considered now simple sequences. You can imagine the improvement it would be to be able to add a new discrete input and output into your program as opposed to having to wire a new relay into an existing system.

A single discrete signal is also referred to as a "bit". Although they sound very simple, bits can be

used in a lot of different ways. For example, by turning a bit on and off you can generate pulses. By using a bit going into a counter you can count the number of times the bit turns on. For example, if you have a motor and each revolution of the motor moves the crane one foot, and you need to move the crane six feet, then you would simply count six pulses. You can also have a bit go into a timer. So if the crane moves one foot per second, then the "crane is currently moving" bit starts a timer that in six seconds tells the PLC that the crane moved six feet. Typically however, discrete signals are used for simple applications. Starting a motor or opening and closing valves. Not glamorous, but very useful

A single bit can have two states – one or zero. Two bits can have four states (00, 01, 10, and 11). Eight bits is known as a byte and can represent 256 states or the numbers from 0 to 255. This is how computers represent numbers and values – by cascading bits. The PLC is no different.

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Analog signals involve detecting different levels of a signal. For example, how fast you want to run a motor. With analog signals, the user can turn a potentiometer that generates a varying voltage or current (analog input) that tells the PLC to send an analog output to the motor indicating the speed that the motor should run. Technically you would need something like a motor inverter between the PLC analog output and the motor to essentially amplify the PLC

analog output to control the motor voltage or frequency and vary the motor speed.

You can represent the position of the potentiometer using 12 bits (0 to 4095). Zero voltage (or current) would be represented by 0 in the PLC and 4095 would represent the maximum voltage (or current). Theoretically (assuming no noise, perfect linearity, and a few other things), the PLC can run the motor at one of 4096 different speeds. Analog signals can be used to bring many types of continuous signals into the PLC – temperature, pressure, level, position, etc. Once in the PLC memory, these signals can be manipulated to control motors, valves, and other field devices.

Let me reemphasize that the function of a PLC is to read these discrete and analog inputs, run some logic based on those inputs, and then write discrete and analog outputs to control the

available environment.

System Architecture

PLCs are continuously expanding in capabilities and the architecture or hardware that is part of that is also

expanding with new functionality. There are some hardware components that are basic to every system. A

PLC system generally has the following components:

1. Processor – consists of the CPU, main memory, possibly some communications and interface capability.

2. I/O – short for Inputs and Outputs – sometimes included in small quantities on a block with the processor

3. Power Supply – Provides power for the various components of the system -sometimes built into a block with the processor.

4. Rack/Chassis – Modular systems generally need a common backplane to plug into. Racks are designed to at least accept a processor and multiple I/O cards. Communication from the processor to the I/O is generally achieved through connections on the rack and power from the power supply is generally distributed through the rack.

5. Communication interfaces – used to communicate with the programmer, operator interfaces, data monitoring tools, SCADA systems, remote I/O, etc. Sometimes built into the processor.

6. Programming Tool – used to program and access the processor.

The processor is generally what most people refer to when discussing a PLC. Many controls specialists

(including myself) tend to interchange PLC, processor and controller when referring to the module that

receives the program and handles processing the logic. All of the components together are generally

referred to as a PLC system. The processor is where a PLC program would reside and is responsible for

receiving data from the input cards and distributing it to the output cards

I/O cards or Input and Output cards are the connections to the real world for the PLC system. As discussed

there are discrete I/O and analog I/O. Both come in many different types. Discrete I/O can come in

various voltages: 24 Volt DC or AC; 120 VAC; TTL; sourcing; sinking; and on and on. Analog I/O has

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even more variety. There are Voltage inputs/outputs, Current inputs/outputs and cards that handle both.

There are cards that just grab inputs or outputs and cards that have a mix of inputs and outputs. There are

cards that are specially designed to connect to Thermocouple and RTD sensors and to compensate for the

non-linearity of the signal. I/O can also be local or remote. Local I/O is located in the same rack or chassis

as the processor, whereas remote I/O can be located in a separate location that is more convenient to wire

into.

The Rack or Chassis is used to house the PLC processor, I/O cards, and other cards that are needed with the

system. Some smaller block style PLC systems don’t use racks and even some modular style systems have

cards that stack together with each card having the required connectors to pass processor communications

and power to each subsequent card.

There are many available communication interfaces ranging from cards designed to communicate via

Ethernet to specialized cards designed to communicate on a manufacturers proprietary communication

link. Generally those cards are used to allow the programmer to interface with the controller, another

device to interface with the controller, or to allow the controller to manipulate and read I/O located in a

remote location.

Programming interfaces vary from handheld programmers (yuck) to more modern programs that can be

installed on a laptop to interface and program the PLC.

Ladder Logic

PLCs are traditionally programmed in a notation known as Ladder Logic or Relay Ladder Logic. This notation is easy for technicians to read and understand. Remember that the first PLC’s were designed to replace large numbers of relays and they were designed to be easy for the electricians responsible for those systems to be able to analyze and troubleshoot. You might want to note that a background and understanding of basic electrical circuits is helpful if you plan to program or work with PLC’s.

An example of a simple motor start/stop circuit is shown below:

Assume that A is the "start" button, B is the "stop" button and C is the output that tells the motor

to run. Imagine power flowing through the ladder logic example. In this case when the operator presses the "Start" button (labeled A above) power flows through the contact labeled A. The [/] contact means "not on" or off. So assuming that the "stop" button (labeled B above) is not pressed then power flows through the B contact and powers the output labeled C and turns on the motor.

Note that we have another contact labeled C below the contact labeled A. This is called a "seal" circuit since contact C "seals" in contact A. Remember the power flow analogy. If it was only the top row: A, B, and C then every time the operator took their finger off the "start" button (contact A) then the motor would stop. But contact C is placed in parallel with contact A so that once the output C turns on then the input C is turned on in parallel with contact A. Therefore the only way to stop the motor is to press the "stop" button (contact B).

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Relay ladder logic - RLL) is the basic PLC logic that converts inputs into outputs. There are several other methods for writing PLC code. Sequential Function Chart (SFC) is excellent for the programming sequential machines -- however most customers hate it. Therefore we usually provide the same framework in RLL. Structured text is another available programming language – much more like a standard high-level language. Function blocks are also available now on many processors. Seen used more in DCS (Distributed Control Systems – a completely new discussion), function blocks are somewhat like a “black box” in electronics. They have a defined set of inputs to the block (don’t get them confused with the PLC inputs) and a defined set of outputs (ditto). The block performs a manipulation of the inputs to elicit appropriate outputs. This can greatly enhance the computational ability of the PLC and simplify some applications. Once again – most clients want to stay in the RLL world. The number 1 rule associated with PLC programming is “keep it simple.” Most end users of PLC’s do not have PLC programmers on staff, or if they do many times those programmers are not the people that are called at 3 am at night when a machine is down. Generally it’s somebody off the maintenance staff that gets pulled in to solve the problem. The simpler the program – the more likely the problem is diagnosed and fixed quickly.

There is an organization that is working to standardize the logic in all PLCs. More information on this can

be found at The PLCopen (IEC 1131-3)

Final Thoughts

PLC Considerations

When selecting a PLC or similar control engine there are many questions:

1. How much I/O? 2. What type of I/O? 3. What type of control logic -- simple ON/OFF or is there PID and data analysis? 4. What type of data is monitored and captured? 5. Are there recipes (databases) involved? 6. Is there an operator interface involved? 7. Are there special communication interfaces required? For example, flow meters, scales,

thermocouples, or other signals that are not a regular discrete or analog signal. 8. Does the application require links to an external network, database, or some type of MES system? 9. Does the application require motion control, bar coding, machine vision, etc?

PLC and / or Computer

Sometimes we do not use a typical PLC for the control engine. Typically what we do is:

1. If the application is small (less than 50 I/O), no databases (only a few choices), and simplistic

HMI then use a PLC. 2. If the application is small, slow (response time greater than 50 milliseconds) and requires

computer functionality (machine vision, networking, databases, multiple axis motion control, etc)

we prefer to do the entire application in Visual Basic (VB). 3. If there are large amounts of I/O (over 100) or you need fast, real-time response, then you will

probably appreciate the PLC handling your real-time and direct I/O tasks and letting the computer

handle the non-real-time tasks (such as HMI, databases, etc). There are a lot of gray areas in

between.

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Although today’s Pentium III running Windows NT or 2000 at 1 GHz with 512 KB RAM is very fast in

comparison to technology only three years ago, it is still nice, in large systems, to use a PLC to help

segment the system functionality. You can write subroutines to segment functionality -- you can also

segment using different controllers.

Frequently Asked Questions (FAQs)

1. What about soft PLCs? Soft PLCs are where the PLC is actually software that resides on a

computer. Although this is way that the industry is headed for the future, as with all new

technology, we would recommend that your first test of new technology not be a critical

application. 2. What brand of PLC is the best? It depends. J If you have a plant full of GE PLCs then it does not

make sense to change to AB. If you have a plant full of some manufacturer that is no longer in

business and you have to switch anyways then it may be time to reevaluate. If you have small

applications that can be linked by computer networks then an AutomationDirect PLC may be fine.

If you have large processes (thousands of I/O) requiring integration of drives and redundancy then

you may want to consider AB or Siemens. 3. How can you program so many different PLCs? Most good PLC programmers, after learning three

different PLC programming languages, can program most any PLC since the basic functions are

the same. In fact there is an International PLC programming standard (IEC 61131-3) that your best

PLC manufacturer’s are adopting.

To learn more

There is a lot you can learn about PLCs. For example, there are books written on PLCs and some of these

books do not seem complete. If you want to learn more, there are many good resources. As always try

searching the best source of information -- the internet.

References:

PLCS.net The History of the PLC by Dick Morley Any large on-line book store such as Amazon or Barnes & Noble will carry books on PLCs Instrument Society of America

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