GE DigitalFlow GS868, Programming Manual

Page: 52 / 171

March 2008
1-44 Programming Site Data
MODBUS Register Map
When equipped with the optional MODBUS output card, the GS868
flow transmitter can send flow data and diagnostic information to a
flow computer (or SCADA) serially, using a Gould-type RTU
protocol. In this case, only the MODBUS function command, 3 (read
multiple registers), 6 (write multiple registers) is valid. The format for
the data exchange is as follows:
•
The send command (initiated by the host flow computer or
controller) comes in the form:
[time delimiter]<Addr><3><First Register MSB>
<First Register LSB><Register Count MSB>
<Register Count LSB><CRC Low><CRC High>[time delimiter]
•
The response (initiated by the host flow computer or controller)
comes in the form:
[time delimiter]<Addr><3><Byte count><Data.........>
<CRC Low><CRC High>[time delimiter]
The format for the returned data types is as follows:
•
Integer (16 bit Integer) <MSB><LSB>
1 Register - 16 bit integer
•
Integer (32 bit IntegerI) <MSB><LSB><LSB><LSB>
2 Register - 32 bit integer
•
Floating Point (FP) <EXP><MAN><MAN><MAN>
2 Registers - 32 bit IEEE floating point number
To request specific parameters from the GS868 via the MODBUS, the
control system must enter the appropriate register number. Only
registers 1 through 90 are available for MODBUS communications,
while registers 508 through 512 are used by the GS868 to store the
MODBUS parameters. For details, see Table 1-12 on page 1-45.