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What they do:
PLC’s quite simply, are computers that are used to control electronic actuators that in turn control machine processes.
Primary types and how they work:
There are several manufacturers of PLC’s and each offers a choice of several models. All of them however, work on similar principles using solid state switching, but different models vary in size and appearance.
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Some may have extra features, or a number of different internal components, or may have a different input and output capacity. They all however contain a basic unit called the CPU (Central Processing Unit), a power supply to operate it, (sometimes incorporated-sometimes separate) and some form of input device to enter program information. In addition, certain models can have add-on units to increase their capacity for input and output channels-obviously to control more equipment and their changes. Because these units are “programmable” they can be loaded with specific commands to carry out tasks on a variety of circuits.
The program can be corrected, modified, or completely changed at anytime, usually without wiring changes. After a program is stored in the memory bank of the CPU it can be duplicated into a nonvolatile storage (hard drive, floppy, etc.) for use in other PLC’s or as a backup. Also, a number of different process programs can be stored and recalled into the PLC’s CPU and will take over control. Most CPU’s have no moving parts and are therefore less likely to develop noticeable service problems. Some do have relays with a mechanical action, yet the actuating circuit malfunction is most likely only traceable by a technician with the proper equipment.
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