What Is a PLC and How Does It Work?

A PLC (Programmable Logic Controller) is an industrial computer that runs a machine or a process according to a program written for it. It reads signals from sensors and switches, decides what to do, and sends commands to pumps, valves, motors and relays. Unlike an office computer, it is built to work in an industrial environment and to respond at a steady rate and in a predictable way.

A PLC works in a repeating loop called the scan cycle. In each cycle the controller reads its inputs, runs the program, and writes its outputs. Inputs are digital signals (a contact, a limit switch) or analog signals (for example 4-20 mA from a level or pressure transmitter), and outputs are the commands that start a pump, open a valve or switch on an alarm lamp.

Most controllers are programmed according to the IEC 61131-3 standard, which defines five programming languages: Ladder Diagram, Function Block Diagram, Structured Text, Instruction List and Sequential Function Chart. Ladder is the most common; it resembles a wiring diagram of contacts and relays, so it is easy to read even for people who are not programmers.

To choose a PLC, check mainly: how many inputs and outputs you need and of which type (digital or analog); which communication protocols are required, for example Modbus or Ethernet; the scan time the process needs; the environment, such as temperature and hazardous area; and which programming software it uses. It is worth starting from the number of inputs and outputs and leaving room for expansion.

In a complete project the PLC does not work alone: it connects to an operator panel (HMI) where the operator sees and controls the process, and sometimes to a SCADA system that gathers data from several controllers. To choose a controller and plan a suitable control panel, contact the Teleris engineering team. This article is general and does not replace the controller manufacturer's manual.