COMMERCIAL IN CONFIDENCE
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2.5
Exception Code Responses
There are 5 possible responses to received messages:
2.5.1.1
Broadcast message
There is never any response to a broadcast message (one with slave address 0). The
message will be acted on if possible, any errors will go unreported.
2.5.1.2
Slave receives incomplete or corrupt message
No response is returned.
2.5.1.3
Slave receives full message but CRC is incorrect
No response is returned
2.5.1.4
Slave receives message correctly, and acts on it correctly
The normal response as detailed under each message heading is returned.
2.5.1.5
Slave receives full message correctly, but cannot act on it
An error response, as detailed under each message heading, is returned as follows
Byte No
Meaning
Value
1
Slave address
xx
2
Original ModBus Function code but with top bit set
8x
3
Error code
xx
4
CRC lo byte
??
5
CRC hi byte
??
The following error codes are implemented :
•
01 - illegal function -
not fully implemented in these versions except for
exit program mode when not in program mode, and for function code 16.
In future releases will be returned on receipt of any function code not
implemented.
•
02 - illegal data address
•
04 - slave device failure
- not currently implemented in these versions ,
but will be returned if the NVram causes problems when reading or writing
•
06 - slave busy - returned if the keyboard is in use when an enter
program mode request is received.
