Yes — in most cases an existing PLC can be connected to the cloud without replacing it or changing its control logic. An edge gateway on the plant network reads selected tags from the PLC over a protocol it already supports — such as Modbus TCP, OPC UA, EtherNet/IP or Siemens S7 — and publishes them securely over MQTT or HTTPS. The connection should be read-only and outbound-only, and the PLC itself should never be exposed to the internet.
The four common approaches
| Approach | How it works | Best for |
|---|---|---|
| Edge gateway reads the PLC | A gateway polls tags over the PLC’s native protocol and forwards them | Most existing installations — no PLC changes |
| PLC’s built-in OPC UA server | Newer PLCs (for example Siemens S7-1500) expose tags via OPC UA to a client | Modern PLCs with OPC UA licensed or enabled |
| Add a communications module | A module adds Ethernet or a protocol the PLC lacks | Older PLCs with only serial ports |
| Read from SCADA or a historian | Take data from the existing SCADA instead of the PLC | Sites that already have SCADA collecting everything |
Protocols by PLC family
- Siemens S7-300/400/1200/1500: S7 protocol, OPC UA on newer models and firmware, Modbus TCP via function blocks.
- Allen-Bradley (Rockwell) CompactLogix / ControlLogix: EtherNet/IP (CIP), OPC UA through add-ons or gateways.
- Schneider Modicon: Modbus TCP natively on most models, OPC UA on newer ones.
- Older or smaller PLCs: Modbus RTU over RS-485 or RS-232, read through a serial gateway.
What to read
Start with a short, agreed list of tags that answer real questions: running states, alarms, counts, levels, flows, pressures, energy and run-hours. Reading hundreds of tags “because they are there” loads the PLC network and buries the useful data. Document each tag with a clear name, unit and description — this naming becomes the foundation of your dashboards and reports.
Security rules
- Read-only by default: the cloud should not be able to write to the PLC unless there is a specific, reviewed need.
- Separate the OT network from office IT, with the gateway as the controlled bridge.
- Unique credentials and certificates per gateway; no default passwords.
- Take a PLC program backup before any work, even when no logic is changed.
- Frameworks such as IEC 62443 are a useful reference for industrial security design.
What it enables
Once plant data reaches the cloud it can feed live dashboards, alarm notifications, production and energy reports, maintenance planning and integration with ERP systems — without touching the control logic that keeps the plant running. See industrial IoT integration and PLC programming.