Choosing a PLC: compact or modular - which fits?
A PLC controls machines reliably, but its form factor decides on headroom and cost. This guide compares compact and modular designs by I/O count, processing power and expandability so you can pick the controller that fits your task.
View controllersCompact or modular - what is the difference?
A compact PLC combines CPU, power supply and a fixed number of inputs and outputs in one housing. A modular PLC splits these functions into separate cards mounted on a rack or backplane bus and freely combined.
The compact design scores with low cost, small footprint and fast commissioning. The modular design offers flexibility instead: you add digital and analogue cards, communication or safety modules exactly as needed and swap single modules during service.
- Compact PLC: everything in one block, fixed base I/O, often a small add-on option.
- Modular PLC: CPU plus freely selected I/O, analogue and bus cards.
- Nano and micro PLCs cover simple control and counting tasks cheaply.
Which metrics drive the selection?
Three figures drive the choice: the number of inputs and outputs (I/O), the required processing power and cycle time, and future expandability. Always plan the I/O with about 20 percent spare so later signals fit without a new purchase.
Processing power is not only about CPU clock but about the time per 1000 instructions and the size of the work memory. Closed-loop control (PID), motion control or fast communication over PROFINET or EtherCAT demand far more power than pure interlock logic.
How do you plan expandability and communication?
A PLC often lives 15 years or more. Choose the form factor for the expected final build, not the first project stage. Modular systems expand almost freely via the backplane bus and via fieldbus couplers.
- Check free slots and the maximum number of modules per rack.
- Mind the available fieldbuses: PROFINET, EtherCAT, PROFIBUS, Modbus TCP.
- Plan decentralised I/O (IO-Link, remote I/O) for distributed machines.
- Budget headroom in the power supply current and in the slots.
Frequently asked questions
When is a compact PLC enough?
For manageable plants up to about 128 inputs and outputs, with simple logic and without demanding control, the compact design is usually the most cost-effective and fastest solution.
What does I/O headroom mean?
You plan extra free inputs and outputs, commonly around 20 percent. Later sensors or actuators can then be wired in without replacing the controller.
Why does cycle time matter?
The cycle time is the duration of one full program pass. Fast processes, closed-loop control and motion control need cycle times below 1 ms, as powerful modular CPUs provide.
Can the program be moved to other hardware later?
Largely yes if it is written to IEC 61131‑3. Vendor-specific function blocks and hardware addresses do need adapting, however.
The right controller for your plant?
We advise on the choice between compact and modular designs and supply controllers, sensors and IO-Link components from a single source.
Standard compliant
Programming to IEC 61131-3 for all common systems.
Scalable
From the nano PLC to the large modular build.
Future proof
Headroom in I/O, slots and fieldbus planned in.
Expert advice
Automation specialists support the sizing.