Wednesday, 7 March 2012

Modbus: What about errors / exceptions

Modbus & Gateways
 
Modbus has a limited way of reporting errors. A server / slave device can respond to a message in a way that reports an error.

These are called exception messages.

If you are looking at a message byte stream, exceptions are easy to identify.






Code     Name / Meaning
1           ILLEGAL FUNCTION

The 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.


2           ILLEGAL DATA ADDRESS

The data address received in the query is not an allowable address for the slave.

3           ILLEGAL DATA VALUE

A value contained in the query data field is not an allowable value for the slave.

4           SLAVE DEVICE FAILURE

An unrecoverable error occurred while the slave was attempting to perform the requested action.

5           ACKNOWLEDGE

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.

6           SLAVE DEVICE BUSY

The slave is engaged in processing a long–duration program command. The master should retransmit the message later when the slave is free.

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