How Does a PLC work?

The working of a programmable logic controller can be easily understood as a cyclic scanning method known as the scan cycle.

Block Diagram of How A PLC Works

A PLC Scan Process includes the following steps

The operating system starts cycling and monitoring of time.
The CPU starts reading the data from the input module and checks the status of all the inputs.
The CPU starts executing the user or application program written in relay-ladder logic or any other PLC-programming language.
Next, the CPU performs all the internal diagnosis and communication tasks.
According to the program results, it writes the data into the output module so that all outputs are updated.
This process continues as long as the PLC is in run mode.

Physical Structure of PLC

The structure of a PLC is almost similar to a computer’s architecture.

Programmable Logic Controllers continuously monitors the input values from various input sensing devices (e.g. accelerometer, weight scale, hardwired signals, etc.) and produces corresponding output depending on the nature of production and industry. A typical block diagram of PLC consists of five parts namely:

Rack or chassis
Power Supply Module
Central Processing Unit (CPU)
Input & Output Module
Communication Interface Module

Rack or Chassis

In all PLC systems, the PLC rack or chassis forms the most important module and acts as a backbone to the system. PLCs are available in different shapes and sizes. When more complex control systems are involved, it requires larger PLC racks.

Small-sized PLC is equipped with a fixed I/O pin configuration. So, they have gone for modular type rack PLC, which accepts different types of I/O modules with sliding and fit in concept. All I/O modules will be residing inside this rack/chassis.

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