Automation and IIoT
What is a PLC (programmable logic controller)?
A PLC is a rugged industrial computer that runs control logic for machines and processes. How PLCs work, IEC 61131-3 languages, and how to get PLC data into MES.
Quick answer
A PLC (programmable logic controller) is a rugged industrial computer that controls machines and processes. It reads inputs from sensors and switches, runs a control program in a fast repeating scan cycle, and drives outputs such as motors, valves and indicators. PLCs are programmed in the IEC 61131-3 languages.
Updated 30 October 2026. Written by the Ajinkya Technologies engineering team, reviewed by Amey Kadle.
PLC explained
PLCs replaced banks of relays in the late 1960s and are now the standard controller for discrete machines and many process units. Each scan the PLC reads all inputs, executes its program and updates all outputs, typically in milliseconds, which makes it predictable and reliable in harsh environments.
The IEC 61131-3 standard defines the common programming languages: ladder diagram, which resembles relay circuits; function block diagram; structured text, a high level textual language; and sequential function charts for step based sequences. Major families include Siemens SIMATIC, Rockwell Allen-Bradley, Mitsubishi, Omron, Schneider and Delta, each with its own engineering software and communication options.
For MES, OEE and traceability, the PLC is usually the best source of truth: it already knows when the machine runs, stops and completes a cycle, and often why it stopped. That data can be read through OPC UA, vendor protocols or MQTT via an edge gateway, read only, without changing the control program. Older PLCs without network ports can still be connected through protocol converters or by tapping simple signals.
Key concepts
- Inputs, program scan, outputs
- IEC 61131-3: ladder, FBD, structured text, SFC
- Major families: Siemens, Rockwell, Mitsubishi, Omron, Schneider
- Best source of run, stop and count data
- Read only connection keeps control logic untouched
Questions
Can old PLCs be connected to an MES?
Usually yes. Older PLCs can be read through serial to Ethernet converters, vendor gateways or by wiring a few simple signals to an IIoT module, without changing the PLC program.
Do we need to change the PLC program to collect OEE data?
Rarely. Run, stop and cycle signals usually already exist in the PLC and can be read as they are. Changes are only needed for data the program does not track.
What is the difference between a PLC and a DCS?
A PLC is a controller often used for individual machines and discrete processes. A DCS is an integrated system of controllers, operator stations and engineering tools, common in continuous process plants.
References
How we help with PLC
We connect PLCs from Siemens, Rockwell, Mitsubishi, Omron and others to MES, OEE and SAP read only, including older models through gateways, without touching the machine program.
Interactive 3D demo with simulated data: cut the network and watch the edge store and replay.
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