PLC Simulator
Modbus exception reference

Modbus Exception Code 03 — Illegal Data Value

The address and function are recognised, 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 recognised, 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

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.