Bringing PLC data to a web interface

Showing a PLC's data in a browser is no longer as hard as it sounds. Classic SCADA software is still at the centre of the job, but for a simple monitoring screen, a phone notification or a summary dashboard for managers, web technologies are very practical.

The basic architecture

PLC → OPC UA server: Many modern PLCs such as the Siemens S7-1500 ship with a built-in OPC UA server. Older devices use a gateway.
Bridge service: A small Node.js or Python service reads tags from OPC UA.
Browser: The service pushes values to the browser live over WebSocket.

// Read a tag every second and send it to every connected browser
setInterval(async () => {
  const value = await session.read({ nodeId: "ns=3;s=Line1.Speed" });
  broadcast({ line1Speed: value.value.value });
}, 1000);

In a real application, OPC UA subscriptions are more efficient than polling every second: the server notifies you when a value changes.

Things to watch out for

Security first. Never expose the PLC network directly to the internet. The bridge should be a controlled link between the OT and IT networks. Don't disable OPC UA's certificate-based encryption.
Separate reading from writing. Monitoring screens should be read-only. Sending commands from the web to a PLC needs separate authorisation and confirmation.
Handle disconnects. When the network drops, the screen should show the last known value and clearly mark it as stale.
Do units and scaling in one place. Resolve whether a raw value is ×10 or ×100 in the bridge so the frontend always works with meaningful values.

This approach doesn't replace SCADA, it complements it: it brings shop-floor data as close as a browser to everyone who needs it.

The basic architecture

PLC → OPC UA server: Many modern PLCs such as the Siemens S7-1500 ship with a built-in OPC UA server. Older devices use a gateway.
Bridge service: A small Node.js or Python service reads tags from OPC UA.
Browser: The service pushes values to the browser live over WebSocket.

// Read a tag every second and send it to every connected browser
setInterval(async () => {
  const value = await session.read({ nodeId: "ns=3;s=Line1.Speed" });
  broadcast({ line1Speed: value.value.value });
}, 1000);

In a real application, OPC UA subscriptions are more efficient than polling every second: the server notifies you when a value changes.

Things to watch out for

Security first. Never expose the PLC network directly to the internet. The bridge should be a controlled link between the OT and IT networks. Don't disable OPC UA's certificate-based encryption.
Separate reading from writing. Monitoring screens should be read-only. Sending commands from the web to a PLC needs separate authorisation and confirmation.
Handle disconnects. When the network drops, the screen should show the last known value and clearly mark it as stale.
Do units and scaling in one place. Resolve whether a raw value is ×10 or ×100 in the bridge so the frontend always works with meaningful values.

This approach doesn't replace SCADA, it complements it: it brings shop-floor data as close as a browser to everyone who needs it.