KROHNE ALTOSONIC V Modbus EN User Manual
Page 15
ALTOSONIC V
ModBus Manual 0300 rev07 E
7.30855.35.00
Page 15 of 64
5.10 Exception Responses
Except for broadcast messages, a master device expects a normal response, when it sends a query to
a slave device.
One of the four possible events can occur from the master’s query:
1.
If the slave device receives the query without a communication error and can handle the query
normally, it returns a normal response.
2.
If the slave does not receive the query due to a communication error, no response is returned. The
master program will eventually process a timeout condition for the query.
3.
If the slave receives the query, but detects a communication error (parity, CRC, LRC), no response
is returned. The master program will eventually process a timeout condition for the query.
4.
If the slave receives the query without a communication error, but cannot handle it, the slave will
return an exception response informing the master of the nature of the error.
The exception response message has two fields that differentiate it from a normal response:
1
the function code field; and
2
the data field.
Ad 1
Function Code Field
In a normal response the slave echoes the function code of the original query in the function code field
of the response. All function codes have a most significant bit of 0.
In an exception response the slave sets the most significant bit of the function code to 1.
The master recognises the exception response by means of this bit and can examine the data field for
the exception code.
Ad 2
Data field
In an exception response the slave returns an exception code in the data field.
This defines the slave condition that caused the exception.
The exception response message:
Header
Slave address
Function
Exception code
Error check
Trailer
Exception codes
Code
Name
Meaning
01
Illegal function
The function code 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.