Editing Modbus (section)

==Exception responses==
When a client sends a request to a server, there can be four possible events for that request:{{Sfn|MODBUS Application Protocol|2012|p=47|ps=, "7 MODBUS Exception Responses"}}

* If server receives the request and execute successfully, server will return a normal response.
* If server cannot receive the request as having communication channel error, server will not respond anything to the client. Client will then have the timeout request error.
* If server receives the request and detect an error on the communication channel (e.g parity, LRC, CRC), server will not response anything to the client. Client will then have the timeout request error.
* If server receives the request and is unable to execute it (e.g client requests to read a non-existent register), server will return an ''exception response'' to client to indicate the nature of the error.
Exception response message includes two other fields when compared to a normal response message:{{Sfn|MODBUS Application Protocol|2012|p=47|ps=, "7 MODBUS Exception Responses"}}

* Function Code: Function code's MSB bit of Exception is 1. This will make this function code 0x80 higher than then request message function code.
* Data: Server returns the exception code inside the Data field. This field defines the nature of the error.

All Modbus exception code:{{Sfn|MODBUS Application Protocol|2012|p=48|ps=, "7 MODBUS Exception Responses"}}
{| class="wikitable"
|-
! Code !! Text !! Details
|-
| 1 || Illegal Function || Function code received in the query is not recognized or allowed by server
|-
| 2 || Illegal Data Address || Data address of some or all the required entities are not allowed or do not exist in server
|-
| 3 || Illegal Data Value || Value is not accepted by server
|-
| 4 || Server Device Failure || Unrecoverable error occurred while server was attempting to perform requested action
|-
| 5 || Acknowledge || Server has accepted request and is processing it, but a long duration of time is required. This response is returned to prevent a timeout error from occurring in the client. client can next issue a ''Poll Program Complete'' message to determine whether processing is completed
|-
| 6 || Server Device Busy || Server is engaged in processing a long-duration command; client should retry later
|-
| 7 || Negative Acknowledge || Server cannot perform the programming functions; client should request diagnostic or error information from server
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| 8 || Memory Parity Error || Server detected a parity error in memory; client can retry the request
|-
| 10 || Gateway Path Unavailable || Specialized for Modbus gateways: indicates a misconfigured gateway
|-
| 11 || Gateway Target Device Failed to Respond || Specialized for Modbus gateways: sent when server fails to respond
|}