West Control Solutions Pro-EC44 - Manual (Page 117)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 110
Link Layer
A query (data request or command) is transmitted from the Modbus Master to the Modbus
Slave. The slave instrument assembles the reply to the master. This instrument is normally a
slave device. It can only act as a master when being use as setpoint master controller to
broadcast its setpoint to other controllers in a multi-zone application.
Figure 62. 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
- the transmitter must not start transmission until 3 character times have elapsed since
reception of the last character in a message, and must release the transmission line within 3
character times of the last character in a message.
Note: Three character times is approximately 0.25ms at 115200 bps, 0.51ms at
57600 bps, 0.75ms at 38400 bps, 1.5ms at 19200 bps, 3ms at 9600 bps and
6ms at 4800bps.
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 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