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PLC-PLC is programmable logic controller. It is solid state device. It is use for control sequential and discrete task in factory environment. We can say it industrial mini computer. Because it  has a central processor unit, memory, input output interface and a programming device.PLC is accept push button ,proximity sensor ,switch,limit switch and other sensors  as a input.and given output with respect to program.output is like valve,motor,LEDs.
Why we choose PLC over Embedded system? Where Embedded system is cheaper then PLC.
Because we can do change and modification in PLC programming and can change whole task as factory environment. But in cheaper  embedded  system  we cannot  do modifications entire task and whole task are made from starting.
PLC History
PLCs was first come in the 1960’s. The most efficient reason for designing PLC  was reduce  the large cost for 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. 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 because of the multitude of moving parts. This also 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 not to mention 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. The problems for maintenance and installation were horrendous.These new controllers also had to be easily programmed by maintenance and plant engineers. 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 use analog signals, meaning that they were now applicable to many more control systems in the world. Unfortunately, the lack of standardization coupled with continually changing technology has made PLC communications a nightmare of incompatible protocols and physical networks. The 1980’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. The 1990’s saw 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. PLCs can now be 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 now switched to a PC based control system.

PLC Types-PLC is three types
1.Compact- In this CPU,i/p and o/p are inbuilt we can not change the i/n and o/p so there are fixed input and output e.g- Micrologix 1000(AB)
2.Compact+Modular-It has CPU ,i/p and o/p. but we can vary its input and output numbers.
3.Modular-It has CPU,i/p and o/p  are different we can add  in CPU like i/p and o/p .   it has cpu i/p and o/p are in different-different parts. We can add input and output in cpu as our need.



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