YSI DIQ/S 182 XT-4-MOD - Manual (Page 127)

System 182 XT-4 Modbus connection
7 - 11 ba76028e01 02/2012
7.5 Modbus query
With each query of sensor data via the Modbus protocol, a block of
consecutive registers is read (up to all of the 16 registers).
For a query of sensor data, determine the first register and the number
of registers to be read (1 ... 16).
Determining the first
register to be read
The first register to be read depends on the sensor number (Sxx) [1 or
2] and the number of the sensor register (R) [1 ... 8]:
(Sxx - 1)] * 8 + R.
Example: Determine the first register with data for the sensor S02:
[(2 - 1) * 8] + 1 = 9
Determining the number
of registers to be read
The number of registers to be read is maximum 16.
Example: Read all data of X sensors
Example of a
Modbus query
and Modbus response
Query of all sensor information of the sensor S02:
Number of sensors
(X) Number of registers
1 8
2 16
Modbus query
Byte Valu
e Information Meaning of the value
1 01h Modbus address of
the DIQ/S 182 XT-4-
MOD 01h --> 1
2 04h Function 04h --> 4
Read Input Register
(see section 7.4.1)
3 00h Start address HI 0008h --> 8
Start with register 9
(Modbus counting method)
4 08h Start address LO
5 00h Number of HI registers 0008h --> 8
8 registers
6 08h Number of LO
registers
7 70h CRC (HI) Checksum (CRC)
8 0Eh CRC (LO)