Establish the wire contract
- Confirm RS485 topology or RS232 context, framing and silent-interval timing.
- Identify unit/slave IDs and supported function codes safely.
- Capture known-good requests/responses and validate CRC.
- Map documented or observed register addresses, including zero/one-based notation.
- Determine signedness, scaling, word order, byte order and multi-register values.
- Test exception responses, timeouts and write consequences.
Gateway or replacement
A gateway can preserve RTU timing/state while exposing a modern interface. Replacement software needs a verified register map and polling/write schedule; aggressive polling or retries can overload old devices. Undocumented registers require controlled experiments tied to known equipment states.
Use traffic capture, field decoding techniques, framing diagnosis and the pillar.
Provide device manuals, topology, captures, unit IDs, required values/commands and safe test access.