What exactly is a PLC?

A PLC basically consists of two elements:
• the central processing unit
• the input/output system

The Central Processing Unit
The central processing unit (CPU) is the part of a programmable controller that retrieves, decodes, stores, and processes information.
It also executes the control program stored in the PLC’s memory. In essence, the CPU is the “brains” of a programmable controller. It functions much the same way the CPU of a regular computer does, except that it uses special instructions and coding to perform its functions.
The CPU has three parts:
• the processor
• the memory system
• the power supply
The processor is the section of the CPU that codes, decodes, and computes data. The memory system is the section of the CPU that stores both the control program and data from the equipment connected to the PLC. The power supply is the section that provides the PLC with the voltage and current it needs to operate.

The Input/Output System
The input/output (I/O) system is the section of a PLC to which all of the field devices are connected. If the CPU can be thought of as the brains of a PLC, then the I/O system can be thought of as the arms and legs. The I/O system is what actually physically carries out the control commands from the program stored in the PLC’s memory.
The I/O system consists of two main parts:
• the rack
• I/O modules
The rack is an enclosure with slots in it that is connected to the CPU.
I/O modules are devices with connection terminals to which the field devices are wired. Together, the rack and the I/O modules form the interface between the field devices and the PLC. When set up properly, each I/O module is both securely wired to its corresponding field devices and securely installed in a slot in the rack. This creates the physical connection between the field equipment and the PLC. In some small PLCs, the rack and the I/O modules come prepackaged as one unit.

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