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PLCs & controllers

How to Choose a PLC: I/O, Protocols and Programming

Choose a programmable logic controller by I/O count and types, expansion, communication protocols, IEC 61131-3 languages, HMI options and support.

Buying guide By the AutomationReviewed editors · Updated · 3 min read · PLCs & controllers

Quick answer

Count your inputs and outputs (digital and analog), add room to grow, and choose a PLC family that expands to cover them. Confirm it speaks the protocols your machines and drives use, such as EtherNet/IP, PROFINET or Modbus TCP. Then check programming software, IEC 61131-3 language support, HMI integration, and the local support and parts availability your team will rely on.

The PLC is the brain of an automated machine. Choose for today’s I/O and tomorrow’s changes.

Step by step

  1. Count I/O: digital inputs and outputs, analog inputs and outputs, high-speed counters.
  2. Add spare capacity for changes.
  3. List protocols used by drives, robots, sensors and your network.
  4. Pick the family that expands to cover it.
  5. Check software: licensing cost, IEC 61131-3 languages, simulation.
  6. Plan the HMI: built-in screen, separate panel or web interface.
  7. Confirm support: distributors, documentation, lead times.

Common protocols

Protocol Common with
EtherNet/IP Many North American installations
PROFINET Many European installations
Modbus TCP and RTU Broad, simple device support
EtherCAT High-speed motion
OPC UA Data exchange with higher-level systems

Programming languages (IEC 61131-3)

Ladder diagram, function block diagram, structured text, sequential function chart and instruction list (deprecated in the current edition).

Connect it to the right robot and the right sensors. Browse PLCs and controllers.

Common questions

What is the difference between a micro PLC and a modular PLC?

Micro PLCs have fixed or limited I/O for small machines. Modular PLCs add I/O, communication and specialty modules for larger systems.

Which programming language should we use?

Ladder diagram is common and familiar to electricians. Structured text suits complex logic. Many teams use both.

Do I need a safety PLC?

If your risk assessment calls for safety functions at a specific performance level, a safety PLC or safety relay may be required.

How much spare I/O should I leave?

Many engineers plan for 10 to 20 percent spare capacity to allow changes and additions.