Function of CPU:
CPU or the central processing unit is the main part of any PLC. The CPU solves the user program logic by using real time input status from input module and updates the status of output module. The CPU consists of – (i) Processor, (ii) Memory.
The processor is a computer that executes a program to perform the operations specified in a ladder diagram or a set of Boolean equations. The processor performs arithmetic and logic operations on input variable data and determines the proper state of the output variables.
The processor functions under a permanent supervisory operating system that directs the overall operations from data input and output to execution of user programs.The processor is responsible for the complete program scan in a PLC. During Program scan processor communicate with the memory.
Memory is used in CPU, are of two types RAM and ROM. RAM memory is used to store the data related to input status, output status, timers, counters, internal bit relay, numerical values etc. ROM memory is to store system program and user program.
Input Modules:
The input module provides the interface between the physical input devices in the real world outside the PLC and the digital arena inside the PLC. The input module has bank of terminals for physically connecting input devices, like push buttons, limit switches etc. The input modules examine the state of physical switches and other input devices and put their state into a form suitable for the processor. The PLC is able to accommodate a number of inputs.
Output Modules:
The input module provides the interface between the physical output devices in the real world outside the PLC and the digital arena inside the PLC. The Output module also has bank of terminals that physically connect output devices like solenoids, motor starters, indicating lamps etc. to a PLC. The role of an output module is to translate signals from the PLC's CPU into a form that the output device can use.
Programming Unit:
The Programmer is a device used to communicate with the circuits of the PLC. The programming unit allows the user to enter and edit the program to be executed.The programming unit is an external electronic package, may be computer or HHT, is connected to the programmable controller when programming occurs.The unit usually allows input of a program in ladder diagram symbols. The unit then transmits that program into the memory of the programmable controller.
Power supply:
i) Power supply is provided to the processor unit, input and output module unit. Power supply may be integral or separately mounted unit. This module can be built in to the PLC processor module or be an external unit. Common voltage levels required by the PLC are 5Vdc, 24Vdc, 220Vac. The voltage lends are stabilized and often the PS monitors its own health.
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