Glossary Updates12 new terms added to the glossaries · October 2, 2026, 22:44 CEST
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Glossary · PLC and control programming

Modular PLC

Also known as: Rack-based PLC, Modular controller

German: Modulare SPS

In industrial automation, a modular PLC is a programmable logic controller built from separate modules, such as a CPU, power supply, digital and analog I/O, communication and technology modules, mounted on a rail or backplane and combined as the application requires.

  • PLC programming

In one sentence

A modular PLC is assembled from separate CPU, I/O and communication modules, so it can scale to larger machines and plants.

Example

A bottling line uses a modular PLC with a CPU, two communication modules, eight digital I/O modules and a high-speed counter module in one rack.

How it applies

  • Engineering: Modular PLCs scale by adding modules and distributed I/O stations. The hardware configuration in the engineering tool must match the physical module order and types; mismatches produce diagnostic errors at startup.
  • Maintenance: Many modular systems allow individual I/O modules to be replaced quickly, sometimes during operation. Whether hot swapping is supported depends on the product and must be checked.
  • Change management: Module types, firmware versions and the hardware configuration are part of the Configuration baseline of the machine.
  • Documentation: Provide a rack or station layout with slot numbers, module order numbers and firmware versions, plus the replacement procedure. Service staff need to know whether a replaced module needs parameters downloaded.

Modular PLC vs. compact PLC

A Compact PLC has built-in I/O and limited expansion. A modular PLC is assembled from modules and scales further. Programming is the same in principle.

By knowledge.aitechdoc.world · Published September 26, 2026 · Last reviewed

Source: AI TechDoc Knowledge editorial definition, based on common PLC hardware practice and IEC 61131-1

Definitions follow the cited standards and specifications. Where a source is a copyrighted publication, such as an ISO, IEC or EN standard, the definition is a close paraphrase, not a verbatim quotation, so as not to infringe copyright. We recommend reading the original publication. The sections “How it applies” are editorial commentary by AI TechDoc Knowledge and are not part of any standard.

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