PLCs were invented in answer to the needs of the automobile industry, and were introduced in the late 1960’s. They were designed to take the place of older, more complicated relay based equipment systems.

ｩ Emil Decker, 2009

Relays were invented in 1835 by Joseph Henry. Relays use an electro- magnet to close or open a switch. This circuit is isolated from the power that runs through the main circuit involved.

ｩ Emil Decker, 2009

This allows a low voltage relay to “throw the switch” on a higher voltage line and keep the operator safe. Relays are used in many electrical systems, including automobiles.

ｩ Emil Decker, 2009

Relays are complicated, and systems using a lot of them tended to be very costly to operate and maintain. If production requirements changed, then the control system had to change also.

ｩ Emil Decker, 2009

The mechanical relays had a limited lifespan, and troubleshooting them were often problematic. Imagine a system using literally thousands of relays.

Typical Relay

ｩ Emil Decker, 2009

By replacing bulky mechanical components with solid state circuits under the control of a microcontroller makes a lot of sense.

ｩ Emil Decker, 2009

The microprocessor is the brain of a PLC system. It consists of the microprocessor, memory, and circuits necessary to store and retrieve data from that memory. It also includes communications ports to other devices.

ｩ Emil Decker, 2009

Processors vary in their capabilities to control devices. Some control as few as 6 inputs and outputs (I/O) and others control as many as 40,000 or more.

ｩ Emil Decker, 2009

One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process.

ｩ Emil Decker, 2009

Ram, or random access memory, is necessary for PLCs. Like computers, this is the area where all instructions are stored during operations. Everything happens in RAM.

ｩ Emil Decker, 2009

ROM, or read only memory, is important too. Instructions burned into ROM cannot be changed. This is where the basic program is stored. You load this and your routines into RAM to run them.

ｩ Emil Decker, 2009

EEPROM, or Electrically Erasable Programmable Read Only Memory, is like ROM, but with the correct equipment, can be reprogrammed, making it more versitile.

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ｩ Emil Decker, 2009

Input modules are simply what they say they are. The input signals, whether via a push button, limit switch, electric eye, or an electrical input .

ｩ Emil Decker, 2009

If there are input modules, then there must be output modules as well. LED readouts, solenoids, relays, DC or AC outputs to machinery, are a few options you have for output.

ｩ Emil Decker, 2009

PLCs, at their simplest, are ON / OFF switches, sequentially operated to send signals to an output device.

ｩ Emil Decker, 2009

Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can cause problems in most solid state control systems, and PLCs are not immune.

ｩ Emil Decker, 2009

PLCs are used in a variety of applications from manufacturing assembly lines to equipment control in the harshest of environments.

Coal mining machine controlled by PLCs

ｩ Emil Decker, 2009

PLC input

modules

PLCprocessor

PLCOutput

modules

• Switches• Sensors• Machine outputs• Other outputs

PLCCommunications

modules

PLC specialPurposemodules

ProgrammableLogic

controller

PLCPowersupply

Standardnetworks

Proprietarynetworks

Smart I/Ointerfaces

Remoteracks

ASCII I/Ointerfaces

• Lamps• Relays• Motor controllers• Solenoid valves• Machine inputs• Other inputs

ｩ Emil Decker, 2009