PLC Simulator
Modbus exception reference

Modbus Exception Code 03 — Illegal Data Value

The address and function are recognized, but a request field or written value is outside the accepted format or range.

03
Load in Modbus sandbox

Exception meaning

Illegal Data Value

The address and function are recognized, but a request field or written value is outside the accepted format or range.

Example exception response PDU

86 03

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

  • Quantity is zero or exceeds the function-specific maximum
  • A command or setpoint value is outside its permitted range
  • Byte count, packed data length or value encoding is inconsistent

Evidence-based checks

  1. 1Check quantity and byte-count fields against the function limit
  2. 2Apply documented scale, signedness and enum values
  3. 3Compare the request against one known-good example

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

Technical reference and worked-example guide

Modbus exception 03 reference: field reference

Direct answer

The reader can distinguish exception 03 from transport silence, illegal function and illegal address, then reduce the request to the rejected value boundary.

Written for pLC, SCADA and integration users diagnosing a valid Modbus request whose contained value or request field the server rejects.

an industrial Ethernet, remote-I/O and IO-Link diagnostics bench with an inspectable controller-to-device signal path while studying Modbus exception code 03 illegal data value diagnosis
System map / 02

NODE 01observable

Definition

Transport success, unit identity, function code, starting address, quantity, payload value, byte order, constraints, device state and raw request and response frames.

NODE 02observable

Signal path

Client request through transport, server parser, function handler, address resolution, value validation, application state and exception response.

NODE 03observable

Worked example

A documented valid request succeeds while one deliberately invalid contained value returns exception 03 reproducibly.

NODE 04observable

Limits

Quantity zero, unsupported coil value, out-of-range register value, paired-register partial write, device mode, broadcast, gateway and firmware change.

NODE 05observable

Common mistake

A client-encoding, transport, unit, function, address, quantity, value, state or server-implementation mismatch.

NODE 06observable

Verification

The corrected request proven against current device documentation and captured raw frames in the target integration.

Answer surface / 07

What does Modbus exception code 03 mean?

The server received and parsed a request but rejected a value contained in it, such as an invalid quantity, encoded output value or device-specific parameter value.

How is exception 03 different from exception 02?

Exception 02 usually rejects the referenced data address; exception 03 rejects a value or request field. Device documentation decides exact implementation limits.

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.