Choosing a PLC for a small machine: what to consider

On a single machine the PLC is just a small box in the control cabinet. But the wrong model can mean rewiring, extra modules or even a different programming package a few months later. Here is what to settle before you order.

1. Count the I/O accurately

List every signal on the machine and split it into four groups:

Digital inputs (DI): push buttons, limit switches, proximity and photoelectric sensors.
Digital outputs (DO): contactor coils, solenoid valves, indicator lamps, buzzers.
Analog inputs (AI): temperature, pressure, load cell, 4–20 mA or 0–10 V signals.
Analog outputs (AO): speed reference for a drive, proportional valve control.

Then add roughly 20–30% spare for signals that appear during commissioning. If the list is not final, choose a family that accepts expansion modules instead of buying the largest CPU on day one.

2. Pick the output type that suits the load

Relay outputs: switch many kinds of load (AC or DC) and are convenient when loads vary, but they are slower and the contacts wear out.
Transistor outputs: fast and durable under frequent switching, and required for pulse output to servos or steppers. Usually DC only and limited current.

If a valve or coil switches every few seconds, prefer transistor outputs (with an interposing relay for larger loads). Also check the supply voltage: most small machines use 24 VDC.

3. Look at the special functions

Some machines need more than on/off logic:

High-speed counters (HSC) for reading encoders.
Pulse outputs for servo or stepper control.
PID control for temperature or pressure.
Real-time clock, recipe storage or data logging.

Check each requirement against the specific model: variants within one family can differ in the number of high-speed inputs or pulse axes.

4. Communication: think ahead about HMI, drives and monitoring

A small machine today typically connects to:

an HMI panel,
a drive or servo,
meters and measuring devices,
a remote monitoring system.

Check that the PLC has RS-485 (Modbus RTU) and/or Ethernet (Modbus TCP or a vendor protocol) and enough ports for the devices above. Having the ports from the start lets you add remote monitoring later without replacing the PLC.

5. Expansion and lifecycle

A family with matching expansion modules lets you add I/O without replacing the CPU.
Ask about availability and lead time for both the CPU and the modules.
Consider the programming software (licensing, supported languages) and which brand your technicians already know. This long-term cost is often overlooked.

Quick checklist

Have you counted DI/DO/AI/AO and added 20–30% spare?
Relay or transistor outputs, and what supply voltage?
Do you need HSC, pulse output, PID or data logging?
Are there enough communication ports for the HMI, drives and monitoring?
Can you add modules later?

Need help choosing a model?

Send us the machine's signal list, load types, the devices to connect and the quantity, and we will work out a suitable PLC configuration with you.

Further reading: PLC selection guides from Industrial Monitor Direct, Moreval and manufacturer datasheets. This article is an original write-up based on field experience.

Cover image: BMA Automation Allen Bradley PLC 3 by Elmschrat Coaching-Blog, licensed under CC BY-SA 3.0, cropped; via Wikimedia Commons.