PLC Simulator
Modbus exception reference

Modbus Exception Code 02 — Illegal Data Address

The function is understood, but one or more requested data addresses are not mapped or allowed.

02
Load in Modbus sandbox

Exception meaning

Illegal Data Address

The function is understood, but one or more requested data addresses are not mapped or allowed.

Example exception response PDU

83 02

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

  • 4xxxx reference notation was sent as the protocol offset
  • The request crosses an unmapped gap
  • The register exists only in a different model, firmware or operating mode

Evidence-based checks

  1. 1Translate the first documented reference into the correct zero- or one-based protocol offset
  2. 2Reduce quantity to one known register
  3. 3Confirm model, firmware and option-dependent map notes

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 register

Technical reference and worked-example guide

Modbus exception code 02 guide: field reference

Direct answer

The reader can use the server’s exception as positive evidence, reconcile reference notation with wire offsets and reduce the request to the first unsupported address.

Written for pLC programmers and technicians diagnosing an exception response that explicitly reports an unsupported register, bit range or request span.

an industrial network diagnostics lab connecting generic controllers, distributed I/O, serial and Ethernet paths to protocol traces and process values while studying Modbus illegal data address diagnosis
System map / 02

NODE 01observable

Definition

Client and server identity, transport, function code, documented reference, zero- or one-based notation, start address, quantity, valid range, data model, access right, device revision and configuration.

NODE 02observable

Signal path

Application request through function and address fields to server map validation, exception response, client log, corrected request, raw data and engineering interpretation.

NODE 03observable

Worked example

One documented address and quantity returns repeatable data that matches a known server value and does not cross an unsupported boundary.

NODE 04observable

Limits

Off-by-one notation, wrong function, range crossing, disabled feature, model variation, firmware revision, remapped gateway, stale manual and replacement device.

NODE 05observable

Common mistake

An identity, model, function, notation, offset, quantity, range, configuration, firmware or documentation mismatch.

NODE 06observable

Verification

The corrected mapping verified against current device documentation, captured requests and responses, known values and failure-recovery tests.

Answer surface / 07

What does Modbus exception code 02 mean?

The server received the request but reports that the addressed data item or requested span is not valid for that device and function.

How do I fix illegal data address?

Confirm exact device and revision, function, reference notation, transmitted offset and quantity, then test one documented item before expanding the request.

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.