Despatch P1400+ Temperature Controller - Manual (Page 64)

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10.2 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.
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