PLC communication: PLC↔PLC, HMI, SCADA, drives, OPC UA
Which mechanism to choose for each communication path of a controller, and the questions to settle before the first connection: data volume, cycle, consistency, ownership and loss behaviour.
Paths and usual solutions
| Path | Common solutions |
|---|---|
| PLC ↔ PLC | Cyclic: PROFINET I-device, EtherNet/IP produced/consumed tags, EtherCAT EAP. Acyclic: S7 PUT/GET, TCP/UDP (TSEND/TRCV), OPC UA client/server or PubSub, Modbus TCP |
| PLC ↔ HMI | Vendor protocol of the panel (S7, CIP), OPC UA, Modbus TCP for third-party panels |
| PLC ↔ SCADA | OPC UA, vendor drivers, Modbus TCP, IEC 104/DNP3 for telecontrol |
| PLC ↔ VFD | Fieldbus telegram (PROFIdrive, CIP drive profile, CiA 402), Modbus RTU/TCP, or hardwired DI/AI |
| PLC ↔ remote I/O | The fieldbus of the I/O system (PROFINET, EtherNet/IP, EtherCAT, PROFIBUS, Modbus) |
| PLC ↔ MES / cloud | OPC UA, MQTT/Sparkplug via an edge device |
Questions to settle first
- Who owns which data — every value has exactly one writer.
- Cycle and volume — 20 words every 10 ms is a different design from 5000 tags every second.
- Consistency — data that belongs together (a recipe, a 32-bit value) must be transferred in one block.
- Loss of communication — each receiver detects it (heartbeat/watchdog counter) and goes to a defined state.
- Data types and byte order — agree INT/DINT/REAL and word order in an interface list.
Heartbeat pattern
(* Sender: increment every cycle *)
TxHeartbeat := TxHeartbeat + 1;
(* Receiver: alarm if the counter does not change for 2 s *)
tCommLoss(IN := (RxHeartbeat = RxHeartbeatOld), PT := T#2S);
RxHeartbeatOld := RxHeartbeat;
CommOk := NOT tCommLoss.Q;
Document the interface
An interface list (tag, direction, type, unit, range, update rate, behaviour on loss) prevents most integration problems between suppliers.
Connections
Open in Knowledge Graph →Loading the map…
Sources
- PROFINET with STEP 7 — Function Manual 11/2022 — Siemens AG
- EtherNet/IP — technology overview — ODVA
- OPC Unified Architecture — overview — OPC Foundation
- MODBUS Application Protocol Specification V1.1b3 — Modbus Organization
Updated 2026-10-02