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How to Choose a PLC

The I/O count, scan speed, communication and expansion criteria to consider when choosing the right PLC for an application.

6 min readUpdated: 2026-07-06

Choosing the right PLC is about balancing today's needs with room for tomorrow's expansion. The criteria below clarify the decision.

Core selection criteria

  • I/O count and type — digital/analog I/O needs + 20% spare
  • Scan speed — low scan time for fast processes (packaging, motion)
  • Communication — PROFINET, EtherCAT, Modbus or EtherNet/IP support
  • Expansion — modular design, additional I/O and special modules
  • Ecosystem — engineering software, local support, spare-part availability
A compact PLC is ideal for small, fixed applications; if the machine will grow, a modular design is more economical long-term.

Common mistakes

  • Sizing I/O exactly at the limit (no spare)
  • Ignoring the existing plant's communication protocol
  • Not assessing spare-part and local-support availability

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