FIELD BUS

Remote input/output stationsIn large installations, remote input/output stations are placed at optimum

locations.• They reduce tremendous materials and labor cost.• Only the communication of main plc with i/o stations is through Network

interface modules by a coaxial cable or a twisted pair of wires must be wired between the remote rack and the processor, replacing hundreds of field wires.

• Also, trouble shooting and connection checks become much easier, since hundreds of wires do not need to be checked all the way back to the master rack.

Any device may interface with the network, because the network is not restricted to only products designed by the network’s manufacturer.

master rack - the rack enclosure containing the CPU or processor module. This rack may or may not have slots available for the insertion of I/O modules.

local rack – it is an enclosure, which is placed in the same area as the master rack, that contains I/O modules. If a master rack contains I/O modules, the master rack can also be considered a local rack.

remote racks - are enclosures, containing I/O modules, located far away from the CPU. Remote racks contain an I/O processor (referred to as a remote I/O processor) that communicates input and output information and diagnostic status just like a local rack with the main processor (CPU).. A remote rack includes a power supply that drives the logic circuitry of the interfaces and a remote I/O adapter or processor module

In an I/O bus network, the PLC drives the field devices directly, without the use of I/O modules; therefore, the PLC connects to and communicates with each field I/O device according to the bus’s protocol.

A BASIC Personal computer interface is generally compatible with the RS-232C, RS-422, and RS-485 communication standards and can communicate to manufacturer’s PLC programming terminal.

Topology:- The way in which a network or systems are physically structured.

Remote subsystems are normally connected to the CPU either in • a daisy chain or bus, multi drop,• ring ( need only a single loop of wire) • Star (requires more wire), configuration.

For a factory environment the bus topology is popular.

complex processes uses Multiple control systems (PLC s). For these controllers to work together, they must communicate.

• Bus:- a bus is simply a collection of lines that transmit data and/or power.

• Network:- A series of points (or devices) connected by some type of communication medium for resource sharing and ease of communication. A computer with a single network interface can communicate with many other computers.

• Network interface module. A special function interface that allows PLC s and other intelligent devices to communicate and transfer data over a high-speed local area communication network.

• I/O bus network. A network that lets input and output devices communicate directly to a PLC through digital as well as supply power to, the field devices that are connected to the bus.

• Node:-A station, such as a personal computer or a PLC, that is connected to a network and can thereby send and receive messages through the network.

types of I/O bus networks-

• device-level bus networks- interface with low-level information devices (e. g push buttons, limit switches, etc.),

• Process bus networks- connect with high-levelinformation devices (e.g., smart process valves, flow meters, etc.), which are typically used in process control applications.

• Process bus networks handle large amounts of data , consisting of information about the process, as well as the status of field devices .

Networking Hardware• Computer (or network enabled equipment)• Network Interface Hardware - The network interface may already be built into the

computer/PLC/sensor/etc. • The Media - The physical network connection between network nodes.

10baseT (twisted pair) is the most popular. It is a pair of twisted copperwires terminated with an RJ-45 connector.

10base2 (thin wire) is thin shielded coaxial cable with BNC connectors10baseF (fiber optic) is costly, but signal transmission and noise properties are very good.

• Repeaters (Physical Layer) - These accept signals and retransmit them so that longer networks can be built.

• Hub/Concentrator - A central connection point that network wires will be connected to. It will pass network packets to local computers, or to remote networks if they are available.

• Router (Network Layer) - Will isolate different networks, but redirect traffic toother LANs.• Bridges (Data link layer) - These are intelligent devices that can convert data on one type of

network, to data on another type of network. These can also be used to isolate two networks.

• Gateway (Application Layer) - A Gateway is a full computer that will direct traffic to different networks, and possibly screen packets. These are often used to create firewalls for security

Advantages of field bus:-• Digital communication allows more than one field

device to be connected to a wire due to addressing capabilities and the device’s ability to recognize data.

• These digital signals are less susceptible than other types of signals to signal degradation caused by electromagnetic interference (EMI) and radio frequencies generated by analog electronic equipment in the process environment

• I/O bus networks have physical advantages as The reduction in the amount of wiring in a plant alone can provide incredible cost savings for manufacturing and process applications

the most popular networks are:• Ethernet/IEEE 802.3 networks, used by PLC CPUs and computers

• device-level bus networks (e.g., CANbus, Seriplex, etc.), which are used by discrete devices

• process field networks (e.g., Fieldbus and Profibus), which are used by analog devices.

• proprietary networks, which are widely used by large PLC manufacturers

COMMUNICATION STANDARDS• Proclaimed standards – These standards are officially established

by the Institute of Electrical and Electronics Engineers (IEEE) and the Electronic Industries Association (EIA) Proclaimed standards are the IEEE 488 instrument bus, the EIA RS-232C, RS-422, and RS-485,

• De facto standards – these communications methods have gained popularity through widespread use. they have no official definition Because they are not properly defined.

some de facto standards cause interface problems; however,the 20 mA current loop, is a good, well-defined de facto standards.

Serial communication :-Data transmitted one bit at a time in series, is called serial communication through simple, twisted-pair cables.

• popular standards for serial communication are the RS-232C and the 20 mA current loop.

• Other PLC standards are the RS-422 and RS-485, which improve performance and give greater flexibility in data communication interfaces.

Parallel communication :- in this data is transmitted multiple bits at the same time over multiple wires. This does allow faster data transmission rates, but the connectors and cables become much larger, more expensive and less flexible. Ex -Centronics printer interface, GPIB/IEEE-488 - (General Purpose Instruments Bus)

Unidirectional,

half-duplex

half duplex :-The data can be sent in either direction, but only in one direction at a time. this is known as half duplex.

full-duplex line:- A communication line used to simultaneously transmit data in two directions.

full-duplex data communication

half-duplex

• RS-232C standard’s electrical characteristic:-At the sending computer an input will either be true or false. The line driver will convert a false value in to a Txd voltage between +3V(5V) to +15V, true will be between -3V(-5V) to -15V. A cable connects the Txd and com on the sending computer to the Rxd and com inputs on the receiving computer. The receiver converts the positive and negative voltages back to logic voltage levels in the receiving computer. The cable length is limited to 50 feet or 15 meters.

• Voltages under –3 V (logic 1) are called mark potentials . • voltages above +3 V (logic 0) are called space voltages.• The area between –3 V and +3 V is not defined• Sometimes a PLC with an RS-232C or RS-422 communication interface must communicate

with an RS-485 device. In this case, an RS-232C–to– RS-485 converter (or an RS-422–to–RS-485 converter) can provide this communication.

Typical data byte:-• before - this is a period where no bit is being sent and the line is true.• start - a single bit to help get the systems synchronized.• data - this could be 7 or 8 bits, but is almost always 8 now. The value is a

byte with the binary value 00010010 (the least significant bit is sent first).• parity - this checks whether the byte was sent properly. The most

common choices here are no parity bit, an even parity bit, or an odd parity bit. In this case there are two high bits . If we are using even parity the bit would be true. If we are using odd parity the bit would be false.

• stop - the stop bits allow a pause at the end of the data. One or two stop bits can be used.

• idle - a period of time where the line is true before the next byte.

1 - DCD (Data Carrier Detect) this indicates when a remote device is present.2 - RXD (receive data) data lines3 - TXD (transmit data) data lines4 – DTR (data terminal ready) handshaking lines 5 - COM (common)6 – DSR (data set ready) handshaking lines 7 - RTS (request to send)8 – CTS (clear to send)9 – RI (Ring Indicator) this is used by modems to indicate when a connection isabout to be made.• Txd, rxd, com are used for transmission reception of data . The remaining

lines called handshaking lines are to be used to detect the status of the sender and receiver, and to regulate the flow of data.

local area networks (LAN):- This networks allows network-connected computers to exchange information among themselves,without having to go through a central location.

A local area network is a high-speed, medium-distance communicationsystem.

For LANs, the maximum distance between two nodes in thenetwork is at least one mile, and the transmission speed

ranges from 1 to 20 Mega baud. Also, most local networks support at least 100 stations, or

nodes.I/O bus network— allows intelligent field devices to communicate

information to PLCs without standard PLC input/output interfaces.

NETWORK PROTOCOLSA protocol is a set of rules that two or more devices must follow if they haveto communicate with each other. Protocol includes everything from the meaning of

data to the voltage levels on connection wires. A network protocol defines how a network will handle the following problems and tasks:

• communication line errors• flow control (to keep buffers from overflowing)• access by multiple devices• failure detection• data translation• interpretation of messages1. Open Systems Interconnection (OSI)-International Standards Organization

(ISO) had published (OSI) reference protocol .2. Institute of Electrical and Electronic Engineers (IEEE) computer society

established the Standards Project 802 for the purpose of developing a local area network standard that would allow equipment from different manufacturers to communicate through a local area network.

The transmission type determines the communication speed and noise immunity.The simplest transmission method is baseband, where voltages are switched off and on

tosignal bit states. This method is subject to noise, and must operate at lower speeds. RS-

232is an example of baseband transmission. Carrierband transmission uses FSK (FrequencyShift Keying) that will switch a signal between two frequencies to indicate a true or falsebit. This technique is very similar to FM (Frequency Modulation) radio where the

frequencyof the audio wave is transmitted by changing the frequency of a carrier frequencyabout 100MHz. This method allows higher transmission speeds, with reduced noiseeffects. Broadband networks transmit data over more than one channel by using multiplecarrier frequencies on the same wire. This is similar to sending many cable televisionchannels over the same wire. These networks can achieve very large transmission

speeds,and can also be used to guarantee real time network access.

INTELLIGENT INSTRUMENTS

In the past few years the development of measurement systems in the form of intelligent devices( known as intelligent instruments, smart sensors and smart transmitters} in which the computational element (usually called a microcomputer or microprocessor) was integrated into the measurement system.

• a microcomputer (computational element) within an intelligent device, take values obtained from one or more sensors and perform a signal processing function digitally that were previously carried out by analogue electronic circuits and provide output at the instrument.

data transfer along the electronic highway by the CPU must include the item of data being transferred and the address where it is being sent.

SMART SENSORS:-Over years the size of intelligent instruments has gradually reduced

and the functions performed have steadily increased. they included a microprocessor within the sensor itself, are usually

called as smart sensors instead of intelligent instruments.Smart sensors are • twice accurate as non-smart devices, • reduced maintenance• costs and require less wiring to the site,• long-term stability is improved, • reducing the required calibration frequency.• Remote calibration capability• Self-diagnosis of faults• Automatic calculation of measurement accuracy and compensation

for random errors• Adjustment for measurement of non-linearities to produce a linear

output• Compensation for the loading effect of the measuring process on

the measured system.

• smart transmitters are also known as intelligent transmitters , also sometimes called multivariable transmitters where it can measure multiple parameters of process.

• Nowadays smart transmitters are routinely bought instead of non-smart versions. However, in many cases, smart transmitters are only used at present in a conventional (non-smart) fashion to give a 4–20mA analogue measurement signal on the two output wires.

Communication with intelligent devices• A number of digital field buses are now used for

instrumentation systems, with protocols such as Profibus and WorldFIP.

• A Prominent protocol is HART (Highway Addressable Remote Transducer).

• The normal requirement for dual analogue/digital communication with an intelligent device is six wires,( two for analog signal, two for command/device status information and two for power supply to the device).

• HART can work on four or even two wires .

HART is an interim network protocol to satisfy communication needs in the transitional period between the use of analogue communication with non-intelligent devices and fully digital communication with intelligent devices according to an international standard digital fieldbus protocol. Because of this need to support both old and new systems, HART supports two modes of use, a hybrid mode and a fully digital mode.

• when HART is used in hybrid mode, the network must be arranged in a star configuration, using a separate line for each field device rather than a common bus.

• In fully digital mode, data transmission of digital as well as status/command signals are carried by one cable to for up to 15 intelligent devices.

HighwayAddressable Remote Transmitter

• Purpose:- to create a way for instruments to digitally communicate with one another over the same two wires used to convey a 4-20 mA analog instrument signal.

• one channel of information – a 4-20 mA DC signal, and another channel for digital communication using pulses of current to represent binary bit values of 0 and 1.

• Digital data is encoded in HART using the Bell 202 modem standard and transmitted by frequency-shift keying, or FSK.

• two audio-frequency “tones” (1200 Hz and 2200 Hz) with an AC current of 1 mA peak-to-peak are used to represent and transmit “1” and “0,”on the 4-20 mA DC signal.

• In HART current loops the total loop resistance must fall within a certain range: 250 ohms to 1100 ohms where as Most 4-20 mA loops contain a single 250 ohm resistor for converting 4-20 mA to 1-5 V. Even loops containing two 250 ohm precision resistors meet this requirement.

• The HART transmitter can be represented as two parallel current sources: one 4-20 mA DC and one AC1mA P-P audio-frequency HART signal along the two wires and turns on and off as necessary to “inject”.

• HART devices are designed to be connected in parallel with each other with out break the loop and interrupt the DC current signal every time to connect a HART communicator device to communicate with the transmitter

• HART Protocol sends and receives digital information across analog wires between smart devices/intelligent field instruments/slave and host systems / control or monitoring system/master.

• A host/master can be any software application from technician's hand-held device or laptop to a plant's process control, asset management, safety or other system using any control platform.

• The HART Protocol communicates at 1200 bps without interrupting the 4-20mA signal and allows a host application (master) to get two or more digital updates per second from a smart field device.

• HART technology is a master/slave protocol, which means that a smart field (slave) device only speaks when spoken to by a master .

• HART Protocol provides for up to two masters (primary and secondary ) such as handheld communicators to be used without interfering with communications to/from the primary master,

• The HART Protocol permits all digital communication with field

devices in either point-to-point or multidrop network configurations .

• point-to-point connection- HART master device is connected to exactly one HART

field device.

• Multiplexer connection-the host/master can communicate with many (> 1000) devices, all with the address zero.

• Multidrop mode- up to 15 field devices are connected in parallel to a single wire pair. The host distinguishes the field devices by their preset addresses which range from 1 to 15.