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A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.
In the late 1960's PLCs were first introduced. The primary reason for designing such a device was eliminating the large cost involved in replacing the complicated relay based machine control systems. Bedford Associates (Bedford, MA) proposed something called a Modular Digital Controller (MODICON) to a major US car manufacturer. Other companies at the time proposed computer based schemes, one of which was based upon the PDP-8. The MODICON 084 brought the world's first PLC into commercial production.
When production requirements changed so did the control system. This becomes very expensive when the change is frequent. Since relays are mechanical devices they also have a limited lifetime which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now picture a machine control panel that included many, possibly hundreds or thousands, of individual relays. The size could be mind boggling. How about the complicated initial wiring of so many individual devices! These relays would be individually wired together in a manner that would yield the desired outcome. Were there problems? You bet!
These "new controllers" also had to be easily programmed by maintenance and plant engineers. The lifetime had to be long and programming changes easily performed. They also had to survive the harsh industrial environment. That's a lot to ask! The answers were to use a programming technique most people were already familiar with and replace mechanical parts with solid-state ones.
In the mid70's the dominant PLC technologies were sequencer state-machines and the bit-slice based CPU. The AMD 2901 and 2903 were quite popular in Modicon and A-B PLCs. Conventional microprocessors lacked the power to quickly solve PLC logic in all but the smallest PLCs. As conventional microprocessors evolved, larger and larger PLCs were being based upon them. However, even today some are still based upon the 2903.(ref A-B's PLC-3) Modicon has yet to build a faster PLC than their 984A/B/X which was based upon the 2901.
Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLCs and they could be far away from the actual machine they were controlling. They could also now be used to send and receive varying voltages to allow them to enter the analog world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. Still, it was a great decade for the PLC!
The 80's saw an attempt to standardize communications with General Motor's manufacturing automation protocol(MAP). It was also a time for reducing the size of the PLC and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers. Today the world's smallest PLC is about the size of a single control relay!
The 90's have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980's. The latest standard (IEC 1131-3) has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time! PC's are also being used to replace PLCs in some applications. The original company who commissioned the MODICON 084 has actually switched to a PC based control system.
Types of PLCs
Programmable Logic Controllers (PLCs) are integrated as either single or modular units.
An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.
A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.
Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.
Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.
Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.
Some of the manufacturers or types of PLCs are given below: anufacturers or Types of PLCs * Allen Bradley PLCs (AB) * ABB PLCs (Asea Brown Boveri) * Siemens PLCs * Omron PLCs * Mitsubishi PLCs * Hitachi PLCs * Delta PLCs * General Electric (GE) PLCs * Honeywell PLCs * Modicon PLCs * Schneider Electric PLCs * Bosch PLCs
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 “rungs” (horizontal lines) as there are control circuits to represent. Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control.[1][2] Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown.
Chapter 5 Creating Relay Logic Diagrams
91
Rule 1.
Input devices are placed near the left corner of the rung.
This means that switches, pushbuttons, and contacts are placed near the left corner of each rung.
Rule 2.
One (and only one) output is placed near the right corner of the rung. This means that one output device such as a motor, a pilot light, or a relay coil must be placed near the right corner of the rung.
Rule 3.
Input devices can be connected in series, parallel, or a com- bination of series and paralle ule 4.
Output devices
Cannot be connected in series. Only parallel output devices can be placed in a rung.
Rule 5.
While input devices can be represented multiple times in the relay logic diagram, each output device can only be represented once in the diagram.
Rule 6.
All the input and output devices must be placed horizon- tally. This means that no vertically positioned input or output devices are allowed in a relay logic diagram
Rule 7.
Current in the relay logic diagram must flow from left to right Rule 9.
The location of each contact associated with a coil can be recorded by the right-hand rail near the coil

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