PLC Simulator
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.

01
Load in Modbus sandbox

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

  1. 1Confirm the exact supported function list in the device manual
  2. 2Verify that the exception function matches the original function with bit 7 set
  3. 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 coils03 Read holding06 Write register02 Illegal address