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Errors & ExceptionsModbus Term

Modbus Exception Response

An error reply from a slave indicating it could not execute a request.

Definition

When a Modbus slave receives a request it cannot fulfil, it returns an exception response instead of normal data. The exception response sets bit 7 of the function code (FC03 = 0x03 becomes 0x83) and appends a 1-byte exception code explaining why it failed. Masters must check for this high bit to detect exception responses before parsing data.

Quick Facts

IndicatorFC byte with bit 7 set (0x80 OR'd)
ExampleFC03 error → 0x83 in response
Exception code1 additional byte

Example

Normal FC03 response starts with: 01 03 …
Exception FC03 response starts with: 01 83 02
                                            │  │
                                            │  Exception code (02 = Illegal Data Address)
                                            FC 0x83 = FC03 | 0x80

Related Terms

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ModPackQT Modbus Glossary · Updated 2026