c3dbfd83_imply_Modbus___Exception_Codes__Meaning
[Exception Code] 05 (05 hex) [Name] Acknowledge [Meaning] 
Specialized use in conjunction with programming commands. The 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.
Simply Modbus - Exception Codes
Meaning