Modbus Mirroring

A modbus module copies a PLC's holding registers and coils across the diode. On the IT side they are served from a Modbus TCP server, so a SCADA system, historian or HMI can read live values without any connection into the OT network.

sequenceDiagram
    participant PLC as PLC (OT)
    participant P as Input box: poller
    participant D as Diode
    participant M as Output box: mirror
    participant C as SCADA / HMI (IT)

    loop every 'interval' (1 s)
        P->>PLC: read holding registers and coils (Modbus TCP)
        PLC-->>P: values
        P->>D: one JSON message (seq n), DYO1-framed UDP
        D->>M: datagrams, reassembled and CRC-checked
        M->>M: write values into its datastore
    end
    C->>M: read registers (Modbus TCP, port_out 502)
    M-->>C: latest mirrored values

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How it works

• Polling. The input box keeps one TCP connection open to the PLC (ip, plc_port 502, unit 1) and reads every configured range each interval seconds. Large ranges are split to respect Modbus limits (125 registers or 2000 coils per request). After any failure it reconnects, and failures are logged once rather than every second.
• Sending. All values from one poll travel as one message, with a sequence number. A message is either received whole and CRC-checked, or dropped whole, so a SCADA client never sees half an update.
• Serving. The output box listens on dyode_out.ip:port for updates and runs a Modbus TCP server on port_out (default 502, all interfaces, or bind_out). Values out of range or outside the configured datastore are rejected and logged.
• Staleness. If no update arrives for stale_after seconds (default 10), the output box logs "no update for Ns, serving stale values". The server keeps answering with the last values, because Modbus has no way to say "stale", so the log is the only signal. Watch for it.
• One way only. A client may write to the output box's server, but those writes only change the mirror. Nothing can reach the PLC.

Address ranges

Ranges are written start-end with the end excluded:

registers:
  - 0-100      # holding registers 0..99
  - 400-450    # 400..449
coils:
  - 0-10       # coils 0..9

DYODE v2: over a serial optocoupler

flowchart LR
    PLC1["PLC 1"] --> POLL["dyode_in.py<br/>one process,<br/>polls every module"]
    PLC2["PLC 2"] --> POLL
    POLL -->|"newline-delimited JSON,<br/>tagged with module name"| OPTO{{"Serial line<br/>through optocoupler"}}
    OPTO --> SPLIT["dyode_out.py<br/>one reader"]
    SPLIT --> M1["Mirror 1<br/>Modbus server"]
    SPLIT --> M2["Mirror 2<br/>Modbus server"]

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DYODE v2 supports only modbus modules. A single process owns the serial port (serial.device, default /dev/serial0, 57600 baud) and multiplexes every module over it. Any other module type in a v2 config is ignored with a warning.