SpiderLive Bridge

Connects OpenPLC (the real PLC runtime, Modbus TCP master) to the SpiderLive web simulator (the virtual plant, in the browser).

[OpenPLC Runtime] ──Modbus TCP──▶ [bridge] ──WebSocket──▶ [SpiderLive browser]
   runs your ladder    master       slave/relay   plant      cylinders + sensors

OpenPLC runs your program and treats SpiderLive as its field I/O: it writes the solenoids (coils) and reads the sensors/buttons (discrete inputs). The bridge exists because a browser cannot speak raw Modbus TCP.

Run it

Requires Node.js 18+. No clone needed — run it straight from npm:

npx spiderlive-bridge

…or install it as a command:

npm install -g spiderlive-bridge
spiderlive-bridge

It starts the Modbus slave (:502) and the WebSocket server (:8080). Ports are configurable: MODBUS_PORT, WS_PORT, MODBUS_HOST, UNIT_ID.

From source (dev):

git clone https://github.com/JohanWences/spiderlive-bridge
cd spiderlive-bridge && npm install
npm start          # the bridge
npm run mock       # (second terminal) a stand-in plant to test the loop

I/O map

Coils — PLC outputs → plant (OpenPLC writes):

| Coil | Signal | PLC addr | |----|--------|------| | 0–5 | Solenoid, cylinder A–F | Q0.0–Q0.5 | | 6 | Signal tower green (running) | Q0.6 | | 7 | Signal tower red (emergency) | Q0.7 |

Discrete inputs — plant → PLC (OpenPLC reads):

| Input | Signal | PLC addr | |----|--------|------| | 0 | START button | I0.0 | | 1 | STOP 1 | I0.1 | | 2 | STOP 2 | I0.2 | | 3 | EMERGENCY | I0.3 | | 4–9 | a1 — extended, cylinder A–F | I0.4–I1.1 | | 10–15 | a0 — retracted, cylinder A–F | I1.2–I8.1 |

Configure OpenPLC

Automatic (OpenPLC Runtime v4)

v4 has no UI for the Modbus master, so the bridge configures it for you. Run once:

npx spiderlive-bridge --setup-openplc

This locates your OpenPLC Runtime install, writes the full I/O map (16 discrete inputs + 8 coils, the table above) into the modbus_master plugin config, and enables the plugin. Then restart the runtime (just restart — don't re-upload) and npx spiderlive-bridge.

The config is the same for everyone — it maps the PLC's whole fixed address space, so any program works as long as it uses %IX0.0–%IX1.5 / %IX8.0–8.1 and %QX0.0–%QX0.7. Only the install path differs per machine, and it's auto-detected (pass --path "<dir>" if it isn't found).

v4 re-disables the master when you re-upload a program. Re-run --setup-openplc after re-uploading (a plain restart keeps it).

Manual (OpenPLC v3 / others)

If your runtime has the Slave Devices UI, add a device pointing at the bridge:

• Protocol: Modbus TCP
• IP: 127.0.0.1 (same machine) · Port: 502 · Slave ID: 1
• Discrete Inputs: start 0, size 16 → mapped to %IX (sensors + buttons)
• Coils: start 0, size 8 → mapped to %QX (solenoids + tower)

Then write your ladder against those %I / %Q and run it. OpenPLC polls the bridge every scan; SpiderLive reacts in real time.

Next step: the SpiderLive browser app connects to ws://localhost:8080 in "Connected (Modbus)" mode — solenoid coils drive the cylinders, and the a0/a1 sensors + buttons are sent back to OpenPLC.