Programmable Logic Controllers are solid state devices designed to perform logical decision-making functions for control applications within the industrial environment. The PLC performs functions that require relays, solid-state electronics, or mini computers. PLC’s are lightweight, rugged devices that operate without fans, air conditioning or electrical filtering. Programmable Logic Controllers have three main components with smaller components in each. These three PLC components are:

The Processor

The processor is the brain of the PLC system. It is a device designed to perform a wide range of production, machine tool, and process-control functions. Conventional electromechanical devices such as relays previously performed these functions. Processors fulfil these same functions, in a much wider scope and variety of control functions, with less effort, making the PLC a much more popular choice.

The processor works using DC power (± 5V), that is supplied by the power supply. Internal DC power is also transmitted through the processor and operates a section of the I/O and devices connected to the service port of the PLC. Once the ladder-diagram program is entered into the processor, it stays the same until changed by the user with one of the programming devices. The program is unaltered through power failure or power off conditions.

The Power Supply

The Power supply for a Programmable Logic Controller converts the input source power into voltages required for internal circuitry. In some cases, it also provides an isolated VDC supply to power DC input circuits, switches and other indicators. The Power Supply of the PLC is an essential component to running the PLC.

The PLC Input/Output (I/O)

Electrical noise, such as spikes in the power lines would have serious impacts on a PLC’s internal circuits since its CPU operates at very low voltages levels. This is where the Input/Output (I/O) portion of a PLC plays an important role. The I/O, both inputs and outputs, protects the CPU from electrical noise. The I/O section is where status signals are filtered to remove noise, validate voltage levels, and CPU decisions are made and put into operation. The PLC Inputs provide their status to a storage area within the CPU and outputs are driven from a similar stored status in the CPU.

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