Siemens SITRANS LR 300, Instruction Manual, Page: 91 / 140

7ML19985CL02 SITRANS LR 300– INSTRUCTION MANUAL Page 89
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Communicat
ions
Error Handling
Modbus Responses
When polled by a Modbus Master, a slave device will do one of the following:
1. Not reply. This 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.
SITRANS LR 300 uses the following exception codes:
Error Handling
Errors can be traced to two general sources:
1. There is an error in transmission.
or
2. The host tries to do something that is not a valid action.
In the first case, the SITRANS LR 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, SITRANS LR 300
will not give an error to the host request. Various actions and the expected outcome are
as follows:
• If the host reads an invalid register, the host will get an undetermined value back.
• If the host writes an invalid register (a non-existing parameter or a read only
parameter), the value will be ignored and no error response will be made. However,
the current value will not reflect the desired new value.
• If the host writes a read only register, then the value will be ignored and no error
response will be made. However, the current value will not reflect the desired new
value.
• If P799 is read-only, then the value will be ignored and no error response will be
made. However, the current value will not reflect the desired new value.
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.
SITRANS LR 300.book Page 89 Monday, June 10, 2002 7:18 PM