Establish the physical link
Identify the exact adapter and port, close competing software, confirm the transceiver direction control, and check A/B polarity against the device vendor’s naming—not just another vendor’s A/B labels.
- One continuous trunk with short stubs
- Termination at the two physical ends
- Biasing in one controlled location
- A reference/common conductor where the installation requires it
Match framing before searching IDs
Baud rate, data bits, parity, and stop bits must match. If framing is wrong, probing 247 Unit IDs adds no information. Once framing is credible, probe a bounded Unit-ID set using one conservative function and address.
Read the evidence in order
No bytes suggests ownership, direction, wiring, ID, or device-state problems. Bad CRC suggests framing or signal integrity. A clean Modbus exception proves the physical and framing layers are working and moves the investigation up to function, address, or quantity.