39
Reference Manual
00809-0100-4825, Rev CB
Section 3: Configuration
March 2014
Configuration
3.5
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 248
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.
Figure 3-3. Typical Multidropped Network
shows a typical multidrop network. Do not use this figure as an installation diagram.
Contact Emerson Process Management product support with specific requirements for
multidrop applications.
Note
Rosemount 248 transmitters are set to address 0 at the factory, which allows 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 and sets it to 4 mA and disables the current
failure mode.
A. Rosemount 248 HART transmitter
E. 4-20 mA
B. Power supply
F. HART interface
C. Power supply impedance
G. Computer or DCS
D. 250
H. Handheld terminal
B
C
H
G
A
E
F
D
