In operating mode Modbus client, the AC500 does only send requests, if the parameters at the MODMAST inputs are logically correct. Nevertheless, it can happen that a server cannot process the request of the client or that the server cannot interpret the request due to transmission errors or in case it’s capabilities are exceeded in any way. In those cases, the server returns an exception response to the client. In order to identify this response as an exception response, the function code returned by the server is a logical OR interconnection of the function code received from the client and the value 80HEX.
General telegram description
|
Error code |
CRC |
|
|
High |
Low |
|
Possible error codes of the client
|
Code |
Description |
|---|---|
|
01DEC |
ILLEGAL FUNCTION The server does not support the function requested by the client |
|
02DEC |
ILLEGAL DATA ADDRESS Invalid operand address in the server or operand area exceeded |
|
03DEC |
ILLEGAL DATA VALUE At least one value is outside the permitted range of values |
|
04DEC |
SERVER DEVICE FAILURE An unrecoverable error occurred while the server was attempting to perform the requested action |
|
05DEC |
ACKNOWLEDGE Specialized use in conjunction with programming commands. The server 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 client. The client can next issue a Poll Program Complete message to determine if processing is completed |
|
06DEC |
SERVER DEVICE BUSY Specialized use in conjunction with programming commands. The server is engaged in processing a long–duration program command. The client should retransmit the message later when the server is free. |
|
07DEC |
NEGATIVE ACKNOWLEDGE Specialized use in conjunction with programming commands. The server cannot perform the programming functions. Client should request diagnostic or error information from server. |
|
08DEC |
MEMORY PARITY ERROR Specialized use in conjunction with function codes 20 and 21 and reference type 6, to indicate that the extended file area failed to pass a consistency check. The server attempted to read record file, but detected a parity error in the memory. The client can retry the request, but service may be required on the server device. |
|
09DEC |
UNDEFINED Actually not defined by Modbus specification but might be used by particular servers. |
|
10DEC |
GATEWAY PATH UNAVAILABLE Specialized use in conjunction with gateways, indicates that the gateway was unable to allocate an internal communication path from the input port to the output port for processing the request. Usually means that the gateway is misconfigured or overloaded. |
|
11DEC |
GATEWAY TARGET DEVICE FAILED TO RESPOND Specialized use in conjunction with gateways, indicates that no response was obtained from the target device. Usually means that the device is not present on the network. |
Example
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Function code: |
01 |
Read n bits |
|
Server operand address: |
4000HEX = 16384DEC |
Area for read access disabled in server |
|
Function code: |
81HEX |
|
Error code: |
03 |