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Guide

How to select a PLC: a practical checklist

Choose a PLC with confidence: count I/O, check communication and safety needs, size memory and scan time, and plan for spares, software and future expansion.

  • 7 min read
  • Updated

The selection checklist

  1. Count inputs and outputs: digital, analogue, high-speed and special signals. Add about 20 percent for spares and future changes.
  2. Decide the control tasks: simple logic, motion, process control, batching or safety. This sets the CPU class.
  3. Check communication: protocols the plant uses (PROFINET, EtherNet/IP, Modbus, OPC UA) and the number of Ethernet ports required.
  4. Estimate program and data memory and the scan time the process needs, including future features.
  5. Decide whether integrated safety is needed. A safety-rated controller can replace separate safety relays.
  6. Check the environment: temperature range, vibration, conformal coating and certifications.
  7. Choose the platform: consider programming software, training, local support and the vendor's lifecycle commitments.

Compact, modular or rack-based?

  • Compact: fixed I/O in one unit, ideal for small machines and cost-sensitive designs.
  • Modular: a CPU with plug-in I/O modules, flexible and the most common choice.
  • Rack-based or large systems: for big plants, redundancy and high I/O counts.

Do not forget

  • Power supply and its capacity
  • Programming software and any licences
  • Memory cards, batteries and communication modules
  • HMI or panel for operators
  • Spare parts for the critical modules

Frequently asked questions

How much spare I/O should I plan?

A common rule is about 20 percent. It costs little at the start and saves a redesign when the machine changes.

Should I choose the same brand as the rest of the plant?

Usually yes. A common platform reduces training, spare parts and software tools, unless a specific requirement points elsewhere.

Do I need a safety PLC?

If your risk assessment requires safety functions such as emergency stop or light curtains, a safety-rated controller or safety modules can simplify the design.

This guide is general engineering guidance. Always confirm part-specific details, ratings and compatibility against the manufacturer’s datasheet. We confirm price, stock and specifications for the exact part in a written quotation.