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Function CodesModbus Term

FC08 — Diagnostics (Loopback)

Diagnostic function to test communication with a slave, including echo (sub-function 0x0000).

Definition

Function Code 08 provides diagnostic utilities. The most common sub-function is 0x0000 (Return Query Data / Loopback Echo), which asks the slave to echo back the data bytes unchanged — confirming the slave is alive and the communication path is functional. Other sub-functions reset counters, clear queues, or report bus statistics.

Quick Facts

FC hex0x08
Sub-function 0x0000Echo / loopback test
Sub-function 0x000AClear Counters
Sub-function 0x000BReturn Bus Message Count

Related Terms

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ModPackQT Modbus Glossary · Updated 2026