March 2008
1-56 Programming Site Data
MODBUS Register Map When equipped with the optional MODBUS output card, the DF868
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 DF868 via the MODBUS, the
control system must access the appropriate register number, as shown
in Table 1-26 on the next page. Only registers 1-108 are available
with the DF868 for MODBUS communications. Registers 508-512
are used by the DF868 to store the MODBUS parameters.
Note: If you request Ch2 or AVE data from a 1-Channel meter, the
values will all be zero.
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