Modbus exception reference

Modbus Exception Code 01 — Illegal Function

The server received a valid request frame but does not support or permit the requested function.

Exception meaning

Illegal Function

The server received a valid request frame but does not support or permit the requested function.

Example exception response PDU

83 01

The exception response function is the original function with bit 7 set. For example, function 03 becomes 83. The second byte is the exception code.

Likely causes

The device does not implement that function code
The function is disabled in the current device state or role
A gateway exposes a narrower function set than the downstream server

Evidence-based checks

1Confirm the exact supported function list in the device manual
2Verify that the exception function matches the original function with bit 7 set
3Try one documented read request before changing transport settings

Run the reference

Turn a protocol table into evidence.

Compose the request publicly, then use Pro for advanced graded exceptions, saved fixtures, complete history and project sharing.

Open sandbox

01 Read coils 03 Read holding 06 Write register 02 Illegal address

Technical reference and worked-example guide

Modbus exception 01 illegal-function guide: field reference

Direct answer

The reader can distinguish exception 01 from silence, invalid address, invalid value and server failure, then preserve enough request and device context to correct the contract.

Written for pLC programmers and technicians diagnosing a Modbus server response that rejects the requested function.

an industrial communications bench used to inspect Ethernet and isolated serial topology, identity, request-response evidence and mapped device data while studying Modbus exception 01 meaning, request evidence and application response

System map / 02

NODE 01observable

Definition

Client and server identity, transport state, transaction context, requested function, server support, operating mode, gateway behavior, exception response and application fallback.

NODE 02observable

Signal path

Client request through transport and server function dispatch to exception 01, client diagnostic, operator information and defined safe application state.

NODE 03observable

Worked example

A documented supported function succeeds against the intended server and returns data matching a known condition.

NODE 04observable

Limits

Unsupported read, unsupported write, read-only mode, gateway translation, model variation, firmware change, server restart and retry.

NODE 05observable

Common mistake

A transport, identity, requested-function, device-capability, mode, gateway or application-response mismatch.

NODE 06observable

Verification

The corrected function verified against current server documentation, captured frames and the intended application result.

Answer surface / 07

What does Modbus exception code 01 mean?

The server received a recognizable request but the requested function is not supported or permitted in the current context.

Is Modbus exception 01 a communications failure?

No. An exception response proves that a server returned a protocol-level rejection; diagnose function support and context rather than only the physical link.

Continue the signal path / 08

Modbus simulator Illegal data address Read holding registers Industrial protocol guide Continue with modbus function 06 single-register write guide Light-curtain muting sequence scenario field guide Simulador Modbus em português

Industrial communications path

Follow the transaction from wire to register

Choose the physical layer, frame the request, inspect the mapped data and diagnose exception evidence in one connected sequence.