Archived:What Do the Various Modbus Exception Codes in Lookout Mean?

Updated Apr 4, 2023

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Reported In


  • Applies to All Modbus Hardware¬†
  • All Lookout Versions

Issue Details

  • I don't know what my Modbus exceptions mean
  • Why am I getting acceptions in Modbus
  • What do my Modbus acceptions mean


Modbus Exception Codes
Exception Code NumberCodeDescription
01Illegal FunctionThe function code received in the query is not an allowable action for the slave. If a Poll Program Complete command was issued, this code indicates that no program function preceded it.
02Illegal Data AddressThe data address received in the query is not an allowable address for the slave
03Illegal Data ValueA value contained in the query data field is not an allowable value for the slave
04Slave Device FailureAn unrecoverable error occured while the slave was attempting to perform the requested action
05AcknowledgeThe slave has accepted the request and is processing it, but a long duration of time will be required to do so. This response is returned to prevent a timeout error from occurring in the master. The master can next issue a Poll Program Complete message to determine if processing is completed
06Slave Device BusyThe slave is engaged in processing a long-duration program command. The master should retransmit the message later when the slave is free.
07Negative AcknowledgeThe slave cannot perform the program function received in the query. This code is returned for an unsuccessful programming request using function code 13 or 14 decimal. The master should request diagnostic or error information from the slave
08Memory Parity ErrorThe slave attempted to read extended memory, but detected a parity error in the memory. The master can retry the request, but service may be required on the slave device