Draw the complete route before testing
Record client IP, gateway IP and port, MBAP Unit ID, downstream serial framing, RTU address, timeout, and expected register. Confirm whether the gateway forwards Unit ID directly, remaps it, or exposes its own management server.
Classify the result before changing settings
Connection refused or unreachable points to the network endpoint. A TCP timeout after connect may be gateway or downstream timing. Exception 0A describes an unavailable gateway path; 0B describes a downstream target that did not respond. Exception 01, 02, or 03 proves a Modbus server answered but rejected the operation.
- Preserve transaction ID and Unit ID
- Record the exact PDU address and quantity
- Distinguish no response from exception response
- Compare gateway and direct-RTU timing when possible
Prove one downstream server and one register
Use a known Unit ID and a short documented read. Avoid simultaneous address scans while testing timing. If direct RTU access works, copy the exact framing and request into the gateway path and account for gateway turnaround in the timeout.
Validate decoded data after the route works
A valid response proves transport and routing, not the engineering interpretation. Only then verify documentation address, function, register width, word order, signedness, scale, and unit. Save the proven route as a connection profile and the decoded values as tags.