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VIPA System 300S
DATA
CRC-CHECK
Byte sequence in the word
Response of the slave
HB130 | CP | 341-1CH01 | en | 18-40
Please consider as soon as you want to access a slave by a writing func-
tion code, you have to write enable the corresponding area by the pro-
tocol parameters with the dialog "Limits".
Here the function code specific data are transferred. More information about the structure
of this field may be found at the function codes.
codes' page 79
n
Message end is identified by means of a 2byte checksum.
–
The first byte to be transferred is the low byte, then the high byte.
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The driver for Modbus Master recognizes message end, when no transmission takes
place during the time period for the transmission of 3.5 times character delay time.
This Time_Out for message end is therefore dependent on the transmission rate:
Baud rate in baud
76800
38400
19200
9600
...
300
For the byte sequence in the word is valid: word = high byte | low byte
If there is no error, the function code is replied.
On recognition of an error in the request message, the slave sets the highest value bit in
the function code (function code OR 80h) of the reply message. This is followed by trans-
mission of one byte of error code.
Slave answer:
OK ® Function code
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Error ® Function code OR 80h & error code
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Communication protocols
Modbus > Modbus Slave - Functionality
Ä Chap. 6.4.8 'Modbus Slave - Function
Time_Out in ms
0.5
1
2
4
...
128
71
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