Most plants already have the data. Temperatures, fill levels, run states, totalisers — it all lives in the PLC or the SCADA historian. The problem is that the people who need it at 7 am on a Saturday are not standing in front of the HMI. Getting that data into a cloud dashboard usually gets framed as a rip-and-replace project. It is not.
The read-only tap pattern#
The safest architectures leave the control layer untouched. Your PLC keeps talking to SCADA exactly as it does today; a small gateway or the PLC's own comms module additionally publishes selected values to the cloud. The tap is read-only: if the cloud link dies, production does not notice.
There are three common ways to build the tap.
Option 1: MQTT from a gateway#
An industrial gateway (or a 30-euro single-board computer in the panel) polls the PLC over Modbus TCP or S7, then publishes JSON over MQTT. One outbound TLS connection on port 8883 — nothing inbound through the firewall.
{
"data": {
"temperature": 71.4,
"fill_level": 82,
"status": "running"
}
}Option 2: REST from existing middleware#
If you already have middleware, a historian, or a line PC collecting data, a scheduled HTTP POST is the shortest path — no new hardware at all.
curl -X POST https://ingest.logable.app/ingest/{deviceId} \
-H "Authorization: Bearer $DEVICE_TOKEN" \
-H "Content-Type: application/json" \
-d '{"data": {"temperature": 71.4, "fill_level": 82}}'Option 3: Direct from modern PLCs#
Newer controllers (and most IIoT-branded comms modules) speak MQTT natively. If your hardware supports it, you can skip the gateway entirely — configure the broker URL, map the tags, and you are publishing.
Choosing between them#
| Path | New hardware | Best when |
|---|---|---|
| MQTT gateway | Small gateway | Firewalled sites, many devices |
| REST | None | Middleware or line PC already exists |
| Native MQTT | None | Modern PLCs with IIoT modules |
Start small, then expand#
Pick one machine and five metrics that answer a real question — "is the line running?", "how full is the silo?". Wire those up end to end, put the dashboard on the wall and on phones, and let the pull for more come from the people using it.