What is a PLC: architecture, I/O and memory
The parts of a programmable logic controller — CPU, memory areas, digital and analog I/O, communication modules and remote I/O — and what they mean for design and troubleshooting.
Components
| Part | Role |
|---|---|
| CPU | Executes the user program cyclically, holds the process image, runs communication |
| Power supply | Usually 24 V DC for CPU and modules; separate load supply for outputs |
| Digital inputs (DI) | Read 24 V DC (or 230 V AC) signals: buttons, limit switches, sensors, contacts |
| Digital outputs (DO) | Switch relays, contactor coils, lamps, valves — transistor (fast, DC) or relay (AC/DC, slow, limited cycles) |
| Analog inputs (AI) | 4–20 mA, 0–10 V, RTD, thermocouple → digital value (e.g. 0…27648) |
| Analog outputs (AO) | Speed references, valve positions |
| Communication | Integrated Ethernet ports, fieldbus masters (PROFIBUS, CAN), serial modules |
| Remote I/O | I/O stations near the machine connected by a fieldbus (PROFINET, EtherNet/IP, EtherCAT) |
Compact PLCs integrate CPU and some I/O in one housing; modular systems mount modules on a rail or rack. Safety PLCs (F-CPUs) run additional certified safety programs and use safety I/O modules.
Memory
- Load memory — the downloaded project (often on a memory card).
- Work memory — code and data used during execution.
- Retentive memory — data kept after power off (counters, operating hours, recipes). See retentive data and startup.
Digital I/O wiring: sinking and sourcing
With 24 V DC sourcing (PNP) devices, the sensor switches +24 V to the input; with sinking (NPN) devices it switches 0 V. Input modules are built for one type (or are configurable); mixing them without care gives inputs that never switch. See discrete sensors.
Choosing I/O
- Count signals per type plus 10–20 % spare.
- Check output current per channel and per module group, and inductive load protection.
- Consider diagnostics (wire break, short circuit) and channel isolation for analog signals from different sources.
- Place remote I/O close to the field to save cable, but keep safety and critical signals on reliable paths.
Connections
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Sources
Updated 2026-10-02