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Errors & ExceptionsModbus Term

Exception 03 — Illegal Data Value

A data value in the request is outside the acceptable range.

Definition

Exception code 03 (Illegal Data Value) means a field in the request data is not allowed. Common causes: writing 0xFFFE to FC05 instead of 0xFF00/0x0000, requesting more than 125 registers in an FC03, or writing a value outside a permitted range. This is not about the register address but about the values themselves.

Quick Facts

Code hex0x03
CauseInvalid data value or quantity in request
FC05 fixUse 0xFF00 for ON, 0x0000 for OFF only

Related Terms

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ModPackQT Modbus Glossary · Updated 2026