A PLC controls equipment: it reads sensors and switches outputs in real time, deterministically, and keeps running without a network. An IoT gateway collects data: it reads PLCs, meters and sensors, buffers the data and sends it securely to cloud platforms. If you need to control a process, you need a PLC. If you need to see and analyse data remotely, you need a gateway. Most modern sites use both — the PLC runs the process and the gateway publishes its data.
Side-by-side
| PLC | IoT gateway | |
|---|---|---|
| Main job | Real-time control and safety interlocks | Data collection and connectivity |
| Timing | Deterministic, millisecond scan cycles | Seconds to minutes is usually fine |
| Without network | Keeps controlling the process | Buffers data until the link returns |
| Programming | IEC 61131-3 (ladder, function block, structured text) | Configuration, Node-RED, scripts |
| Protocols | Fieldbuses, Modbus, EtherNet/IP, PROFINET | Modbus, OPC UA, BACnet in; MQTT, HTTPS out |
| Cloud connectivity | Limited or none on most models | Designed for it — TLS, certificates, GSM |
| Typical cost | Higher, with I/O modules | Lower |
When a PLC is the right choice
- Starting, stopping and sequencing pumps, motors, valves or conveyors.
- Safety interlocks, duty/standby rotation and dry-run protection.
- Anything that must keep working if the internet or server is down.
When an IoT gateway is the right choice
- Reading meters, generators and sensors that already exist.
- Getting data from an existing PLC or SCADA system into a cloud dashboard.
- Remote sites where only monitoring and alerts are needed.
Why most sites use both
The PLC and gateway do different jobs. Keeping control local in the PLC means the process is safe and predictable regardless of connectivity, while the gateway provides the remote visibility, history and integration that management and maintenance need. Some devices blur the line — PLCs with MQTT support, gateways with basic logic — but the principle of keeping critical control local still applies.