KROHNE ALTOSONIC V12 EN - Manual (Page 120)

10
MODBUS PROTOCOL DESCRIPTION AND SET-UP
120
ALTOSONIC V12
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If the request is not applicable, an exception response will be sent.
For more information, refer to
Exception responses on page 120.
10.5.10 Exception responses
Except for broadcast messages, a master device expects a normal response, when it sends a
request to a slave device. One of the four possible events can occur upon the master device’s
request:
• If the slave device recieves the request without a communication error and can handle the
request normally, it returns a normal response.
• If the slave does not receive the request due to a communication error, no response is
returned. The master program will eventually process a timeout condition for the request.
• If the slave receives the request, but detects a communication error (parity, CRC, LRC), no
response is returned. The master program will eventually process a timeout condition for the
request.
• If the slave receives the request without a communication eroor, 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.
Function Code Field
In a normal response the slave echoes the function code of the original request in the function
code field of the response. 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.
Data Field
In an exception response the slave returns an exception code in the data field. By means of this
exception code the slave reports a reason for not being able to respond normally.
The exception response message:
Header Slave
address Function Starting
address Quantity
of points Error
check Trailer
-- 11(h) 10(h) Hi
9C(h)
Low
41(h)
Hi
00(h)
Low
02(h)
-- --
Table 10-8: Response multiple holding registers
Header Slave address Function Exception code Error check Trailer
Table 10-9: Exception response message