Programmable Logic Controllers

Rugged industrial computers that run machines and processes using ladder logic and other IEC 61131-3 languages.

Intermediate

A PLC reads inputs (sensors, switches), executes a program in a fast repeating scan cycle (read inputs → execute logic → write outputs), and drives outputs (contactors, valves, VFDs).

IEC 61131-3 languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC) and Instruction List (deprecated).

A classic motor start/stop seal-in rung:

|--[ START ]--+--[/ STOP ]--[/ OVERLOAD ]--( MOTOR )--|
|             |                                       |
|--[ MOTOR ]--+                                       |

PLCs connect to HMIs and SCADA over Modbus, PROFINET, EtherNet/IP and others. Major brands include Siemens, Allen-Bradley (Rockwell), Schneider, Mitsubishi and Delta.

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