Hardware Components of a PLC System
Processor unit (CPU), Memory, Input/Output, Power supply unit, Programming device, and other devices.

Central Processing Unit (CPU)
CPU – Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.
CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be discovered early.

System Busses
The internal paths along which the digital signals flow within the PLC are called
busses.
The system has four busses:
- The CPU uses the data bus for sending data between the different elements,
- The address bus to send the addresses of locations for accessing stored data,
- The control bus for signals relating to internal control actions,
- The system bus is used for communications between the I/O ports and the I/O unit.

Memory
System (ROM) to give permanent storage for the operating system and the fixed data used by the CPU.
RAM for data. This is where information is stored on the status of input and output devices and the values of timers and counters and other internal devices. EPROM for ROM’s that can be programmed and then the program made permanent.

I/O Sections
Inputs monitor field devices, such as switches and sensors.
Outputs control other devices, such as motors, pumps, solenoid valves, and lights.

Power Supply
Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electrical supply as a separate module, while small and medium series already contain the supply module.

Programming Device
The programming device is used to enter the required program into the memory of the processor.
The program is developed in the programming device and then transferred to the  memory unit of the PLC.


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