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Errors & ExceptionsModbus Term

Exception 01 — Illegal Function

The slave does not support or recognize the requested function code.

Definition

Exception code 01 (Illegal Function) means the slave does not implement the requested function code, or the function code is not applicable to the device type. Common causes: requesting FC23 or FC43 from a device that only supports basic read/write, or sending a vendor-specific function code to the wrong device.

Quick Facts

Code hex0x01
CauseUnsupported or unknown function code
FixCheck device datasheet for supported FCs

Related Terms

Try it in ModPackQT

See Exception 01 — Illegal Function in action — poll registers, write values, and explore function codes in a free browser-based Modbus tool.

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ModPackQT Modbus Glossary · Updated 2026