Rosemount 248, Reference Manual

Page: 38 / 80

Reference Manual
00809-0100-4825, Rev BB
August 2005
Rosemount 248
3-14
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 375 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
Figure 3-3 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, 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.
Power Supply
Power Supply
Impedance
Handheld Terminal
Computer or DCS
Rosemount 248
HART transmitter
4–20 mA
HART Interface
250
Ω