User's Manual

PL-611 IQ Radar 300 Page 87
Communications
Communications Communications
Communications
Error Handling
Modbus Responses
When polled by a Modbus Master, a slave device will do one of the following:
1. Not reply. Which means that something went wrong with the transmission of the
message.
2. Echo back the command with the correct response (see the Modbus specification for
more details). This is the normal response.
3. Return an Exception Code. This reflects an error in the message.
IQ Radar 300 uses the following exception codes:
Code Name Meaning
01 Illegal Function
The function code received in the query is not
an allowable action for the slave.
02 Illegal Data Address
The data address received in the query is not
an allowable address for the slave.
03 Illegal Data Value
A value contained in the query data field is not
an allowable value for the slave.
Error Handling
Errors can be traced to two general sources:
5. There is an error in transmission.
or
oror
or
6. The host tries to do something that is not a valid action.
In the first case, the IQ 300 does not respond and the master waits for a Response Time
Out error, which causes the master to re-send the message.
In the second case, it depends on what the host tries to do. In general, IQ 300 will not
give an error to the host request. Various actions and the expected outcome are as
follows:
• the host reads an invalid register, the host will get an undetermined value back.