YSI IQ S ENSOR N ET Field bus linking - Manual (Page 27)

IQ S ENSOR N ET Field bus linking Communication with field buses
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4.2 Communication with Modbus RTU, Modbus TCP
4.2.1 Data transmission
On each query of sensor data with the Modbus protocol, a block of con-
secutive registers is read.
Query and response
telegram
Data transmission takes place via query and response telegrams. The
form of the telegrams is determined by the Modbus RTU protocol.
The Modbus master transmits a query telegram with a Modbus com-
mand to the Modbus slave (here, the IQ S
ENSOR N ET ). The Modbus
slave transmits a response telegram with the requested data or an error
message.
4.2.2 Query format
Supported
Modbus commands
The IQ S ENSOR N ET provides the data block for read access only. Writ-
ing Modbus commands are not carried out.
The following commands are available:
For a query of sensor data, determine the first register and the number
of registers to be read.
Determining the first
register to be read
The first register to be read depends on the sensor number (Sxx):
R = [(Sxx - 1) * 8] + 1.
Example: Determine the first register with data block for the sensor
S02:
R = [(2 - 1) * 8] + 1 = 9
The number of registers to be read for one sensor data block is 8 reg-
isters.
With one Modbus query, up to 125 registers (15 data blocks) can be
read.
Modbus commands Command code
Read Input Register 04h
Read Holding Register 03h