West Control Solutions 8700+ - Manual (Page 127)

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Modbus Communications 59305, Issue 7 – March 2014
Link Layer
A Query (or command) is transmitted from the Modbus Master to the Modbus Slave. The
slave instrument assembles the reply to the master. All of the instruments covered by this
manual are slave devices, and cannot act as a Modbus Master.
Figure 40. Modbus Link Layer
A message for either a QUERY or RESPONSE is made up of an inter-message gap followed
by a sequence of data characters. The inter-message gap is at least 3.5 data character
times.
Data is encoded for each character as binary data, transmitted LSB first.
For a QUERY the address field contains the address of the slave destination. The slave
address is given together with the Function and Data fields by the Application layer. The
CRC is generated from the given address, function and data characters.
For a RESPONSE the address field contains the address of the responding slave. The
Function and Data fields are generated by the slave application. The CRC is generated from
the address, function and data characters.
The standard MODBUS RTU CRC-16 calculation employing the polynomial 2
16
+2
15
+2
2
+1 is
used.
Inter-message
gap Address
1 character Function
1 character Data
n characters CRC Check
2 characters
MODBUS
MASTER
SLAVE
INSTRUMENT
QUERY
RESPONSE