Preserve bus behavior
- Document two-/four-wire topology, termination, biasing and node locations.
- Capture passively without changing loading or direction control.
- Identify master polling, node addresses and response timing.
- Record driver-enable turnaround and silent gaps on half-duplex links.
- Detect collisions, echoes and contention before blaming framing.
Infer the application layer
Group frames by address and request/response timing, then test length, command, payload, counters and checksums. Change one device state at a time and validate hypotheses across different nodes.
Use serial protocol decoding, capture principles, Modbus diagnosis when applicable and the pillar.
Provide topology, transceiver/interface details, directional captures, node states and safe test access.