Establishing Multidrop Communication - Emerson Rosemount 644 HART Reference Manual

Reference Manual
00809-0100-4728, Rev MB
2.12

Establishing multidrop communication

Multidropping refers to the connection of several transmitters to a single communications transmission
line. Communication between the host and the transmitters takes place digitally with the analog output
of the transmitters deactivated.
Many Rosemount transmitters can be multidropped. With the HART communications protocol, up to 15
transmitters can be connected on a single twisted pair of wires or over leased phone lines.
A Field Communicator can test, configure, and format a multidropped Rosemount 644 Transmitter in
the same way as in a standard point-to-point installation. The application of a multidrop installation
requires consideration of the update rate necessary from each transmitter, the combination of
transmitter models, and the length of the transmission line. Each transmitter is identified by a unique
address (1–15) and responds to the commands defined in the HART protocol. A HART-based
communicator can test, configure, and format a multidropped Rosemount 644 Transmitter the same as
in a standard point-to-point installation.
Note
Multidrop is not suitable for safety-certified applications and installations.
Figure 2-24. Typical Multidropped Network
A.
H.
A. Power supply
B. Power supply impedance
C. 250 
D. Handheld terminal
E. Computer or DCS
F. HART Interface
G. 4-20 mA
H. Rosemount 644 HART Transmitter
Note
Rosemount 644 Transmitters are set to address 0 at the factory, allowing them to operate in the
standard point-to-point manner with a 4–20 mA output signal. To activate multidrop communication,
the transmitter address must be changed to a number between 1 and 15. This change deactivates the 4–
20 mA analog output, sending it to 4 mA. The failure mode current also is disabled.
Configuration
D.
B. C.
F.
G.
E.
Configuration
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