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95
SCADA-Modbus® System Guidelines
D.10 Query
The Modbus ASCII query must take the form shown below that specifies the starting
register and number of registers to be read:
For example, to read the level channel of the 940 Flow Meter, the query must be as
indicated in
Table 18 .
The master queries the flow meter using a Read Holding Registers request, which implies
a 4XXXX register reference, to slave device address 01. The message requests data from
holding registers 40007–40008 to obtain the level information, which requires two
registers to store the floating point value. Note that registers are referenced from zero in
the data field.
CFM 17 DEGREE_F 42
CFH 18 MILS 43
CFD 19 VOLTS 44
CMS 20 FPS 45
CMM 21 MPS 46
CMH 22 PCT_O2 47
CMD 23 PCT_H2S 48
GPS 24 PCT_LEL 49
GPM 25 VDC 50
Table 17 SCADA-Modbus Units of Measure Codes
Unit Code Unit Code
Start
‘:’
Slave
Address
Function
(03)
Start
Address
High
Start
Address
Low
No. of Pts.
High
No. of.
Pts. Low
LRC <CR> <LF>
Table 18 Channel Query to Read Level (Example)
Start ‘:’
Slave Address 01
Function 03
Starting Address High 00
Starting Address Low 06
No. of Registers High 00
No. of Registers Low 02
LRC F4
Stop <CR><LF>