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plc_bridge

PLC writes typed data to /dev/shm. Go reads it, holds a snapshot, and re-publishes it as Modbus TCP (and later: HTTP/WebSocket for Svelte).

The PLC has no Modbus code. There is no register table inside CODESYS. The register layout lives in Go and is one file: internal/modbus/addresses.go.

Why

Putting Modbus inside the PLC is what we did before (Device/Application/Programs/ModbusProgram — legacy, frozen). Maintaining a flat 16-bit register array next to the control logic forced manual offset arithmetic, alignment hazards, and two sources of truth whenever a new variable was added.

This bridge makes the PLC publish typed state — one struct, with named fields. Go is the only place that knows about Modbus addressing, JSON, WebSocket, etc. Adding a new external interface never touches the PLC.

Topology

  CODESYS RT task                     external SCADA / HMI
        |                                       |
        | writes PlcData (seqlock)              | Modbus TCP (:5020)
        v                                       v
  /dev/shm/plc_data            +----------------------------+
  /dev/shm/plc_cmd  <--------- |   plc_bridge (Go)          |
                               |   - shm reader (10 ms)     |
                               |   - state.Snapshot         |
                               |   - modbus tcp server      |
                               +----------------------------+

PLC and Go agree on two things: the byte layout of PlcData and PlcCommand, and their (magic, version) headers. Everything else is internal to one side.

Layout contract

File Side Role
../codesys_export/Device/Application/ PLC DUTs + GVL + publisher POU, auto-exported from the CODESYS IDE. One file per IDE object.
internal/shm/layout.go Go mirror DUTs

Both files declare Magic, Version. Bump Version on every layout change, in both files. The Go reader refuses to mount a segment whose version it does not recognise, instead of silently reading garbage.

Adding a new published variable

  1. Add the field to a payload struct (MachineState, MotionState, or a new domain struct) in:
  2. Bump PLC_DATA_VERSION / PlcDataVersion in both.
  3. If you want it on Modbus, add an address constant + an EncodeData line in internal/modbus/addresses.go.

Adding a new write (Modbus master → PLC)

Same pattern, but on PlcCommand and commandFields in internal/modbus/addresses.go.

Deploy

make deploy   # build web + go, rsync, sudo install, systemctl restart
make rollback # revert to previous binary
make logs     # follow journald

Remote one-time setup

shm permissions (/etc/systemd/system/plc_bridge.service.d/shm-perms.conf)

CODESYS creates shm files as rwxr-x---. This drop-in chowns them to 664 before plc_bridge opens them, so plc_bridge user (in root group) can write plc_cmd.

[Service]
ExecStartPre=-+/bin/chmod 664 /dev/shm/plc_data /dev/shm/plc_cmd

After creating: sudo systemctl daemon-reload

shm layout versions

Segment Size Version Direction
plc_data 88 B v2 PLC → Go → HMI
plc_cmd 48 B v1 HMI → Go → PLC

Version mismatch → plc_bridge logs error and exits (Restart=on-failure retries until PLC is updated).

Running

# 1. PLC must already be publishing /dev/shm/plc_data and /dev/shm/plc_cmd.
# 2. Pin to a non-isolated CPU (PLC owns CPU 2-3).
taskset -c 0 go run ./backend/cmd/plc_bridge

Flags: --modbus :5020 --http :8080 --poll 10ms --push 100ms

What this folder is NOT

  • Not a Modbus master — only a slave.
  • Not a control loop — control stays in the PLC.
  • Not a multi-tenant gateway — assumes one PLC instance per process.