PLC Simulator
Modbus exception reference

Modbus Exception Code 04 — Server Device Failure

The server accepted the request format but encountered an unrecoverable problem while attempting to execute it.

04
Load in Modbus sandbox

Exception meaning

Server Device Failure

The server accepted the request format but encountered an unrecoverable problem while attempting to execute it.

Example exception response PDU

83 04

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

  • Internal device application fault
  • Requested operation depends on unavailable hardware or state
  • Gateway or server cannot complete the downstream operation

Evidence-based checks

  1. 1Preserve the exception, device status and timestamp
  2. 2Read documented diagnostic or status registers if safe
  3. 3Review device logs and manual before resetting or repeating writes

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