MES Integration Series · Part 1
PLC-to-MES Communication Architecture
How do I connect a PLC to the MES securely and reliably?
Between a PLC and an MES there are usually three or four steps: the machine level, an edge gateway, the factory network and the MES server, with the ERP above. This page shows what each layer does, helps you choose between OPC UA, MQTT, REST and a database write, lets you test firewall rules between the zones and shows why subscriptions load the system less than polling. The numbers are examples.
Data climbs through layers. Between the layers the protocol changes: the machine speaks its native protocol, the edge converts it to a standard one, and the MES and ERP talk through IT protocols. Click a layer.
| Protocol | Type | How it works | Good for | Pros | Cons |
|---|
The scores come from the answers you give and from the usual strengths and weaknesses of each protocol (see the table). They are a starting point for the discussion with your IT and automation teams, not a verdict.
The network is split into zones. The OT zone (levels 0 to 2) holds machines, PLCs and robots. The DMZ (level 3.5) sits between OT and IT and holds the gateway and the firewall. The IT zone holds the MES server, the business applications and the ERP. The rule tester applies a simple policy: no direct path between IT and OT, and only the allowed protocols between neighbouring zones. Real rules come from your IT and OT teams.
Polling asks for every tag at a fixed interval, whether it changed or not. A subscription (OPC UA) or a publish on change (MQTT) sends only what changed. The change rate is an assumption: use the real one from your machine. The capacity figure is an assumed gateway limit.
Tick the practices you already follow.
| Stage | Tools shown in the case | What they do here |
|---|
The tools are examples, not recommendations. Choose by what your team owns and what your machines and PLCs support.
The examples are simplified. Design the real network and security with your IT and OT teams and follow your own policies and the relevant standards.