Processor Unit - PLC

Processor unit houses the microprocessor, memory and communications circuitry necessary for the processor to operate and communicate with the I/O and peripheral equipment. The DC power required for the processor is provided either by a power supply that is integral part of the processor unit or by separate power supply unit. Processor is “Brain” of plc which is decision maker that controls the operation of equipment to which it is connected. It also controls the operation of output devices that are connected to output modules based on the status of the input devices and the program that has been entered into memory.

Processors control as few as 8 or as many as 40000 real word inputs and outputs. The larger the number of input and output devices that are required for the process, the more powerful the processor must be to properly control the number of I/O that will be connected.

The processor may be self-contained unit or may be modular in design. Processor scan does the following:

1. Determine status of input devices

2. Interpret logic of program (read and solve ladder logic)

3. Update (Turn “ON” or “OFF”) output devices

4. Communication with connected devices and housekeeping (Update internal timers, counters and   registers)

Normally, before any output devices can be turned ON or OFF, The processor has to scan the entire program that is in user memory. The program may be only of few rungs or it may be hundreds of pages in length, depending upon the equipment that is being controlled. Some input devices operate so  fast that by time the user program can be read and solved and outputs updated, the input device may be changed positions more than once since the processor originally determined its status at the start of the scan. It may be true for an output device that may be updated sooner than as regular scan will allow. To solve the problem, Many PLCs have special program instructions that allow critical or high speed Input and output devices to be actually interrupting the scan when it is reading the program and allow I/O devices to be updated immediately.

Example for collecting bottles at required quantity at required time is shown below……

We study about Memory, Memory chip and is application in next blog.