Modbus HEX Diagnostics: Why Raw TX and RX Data Matters
Published September 25, 2026
Why inspecting raw Modbus request and response bytes can shorten troubleshooting time during commissioning.
When a Modbus test fails, the raw request and response often tell you more than a single error message. HEX output lets an engineer see what was actually transmitted and received at the communication layer.
What to compare
Look at the device address or unit identifier, function code, starting address, quantity or value, and response length. For RTU, also consider the CRC. For TCP, inspect the MBAP header and protocol fields.
Use a known-good request
Start with one request from the device documentation. If the device answers a known-good request but not your configured request, narrow the difference to address, function, quantity or formatting.
Why this helps
HEX diagnostics are especially useful when integrating a new meter, PLC, VFD, controller or gateway because they make assumptions visible. M_Logger 2.0 includes HEX wire logging for this kind of inspection.