Multidrop Communication - Emerson Rosemount 3051 Reference Manual

Section 3: Configuration
November 2012
3.14

Multidrop communication

Multidropping transmitters 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. With smart
communications protocol, up to fifteen transmitters can be connected on a single twisted pair
of wires, or over leased phone lines.
Multidrop installation requires consideration of the update rate necessary from each
transmitter, the combination of transmitter models, and the length of the transmission line.
Communication with transmitters can be accomplished with HART modems and a host
implementing HART protocol. Each transmitter is identified by a unique address (1–15) and
responds to the commands defined in the HART protocol. Field Communicators and AMS
Device Manager can test, configure, and format a multidropped transmitter the same way as a
transmitter in a standard point-to-point installation.
Figure 3-13
diagram.
Note
A transmitter in multidrop mode has the analog output fixed at 4 mA. If an LCD display is
installed to a transmitter in multidrop mode, it will alternate the display between "current fixed"
and the specified LCD display output(s).
Figure 3-13. Typical multidrop network
The 3051 is set to address zero (0) at the factory, which allows operation 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 from 1 to 15. This change deactivates the
4–20 mA analog output, sending it to 4 mA. It also disables the failure mode alarm signal, which
is controlled by the upscale/downscale switch position. Failure signals in multidropped
transmitters are communicated through HART messages.
68
shows a typical multidrop network. This figure is not intended as an installation
HART Modem
Power
Supply
Reference Manual
00809-0100-4001, Rev JA
Configuration