22
Reference Manual
00809-0100-4022, Rev GA
Configuration
September 2016
Configuration
2.11
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. Up to 15
transmitters can be connected on a single twisted pair of wires.
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 Bell 202 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 Manger can test, configure, and format a multidropped transmitter the same way as
a transmitter in a standard point-to-point installation.
shows a typical multidrop network. This figure is not intended as an installation
diagram.
Figure 2-3. Typical Multidrop Network
A. Bell 202 Modem
B. RS-232-C
C. Power supply
The Rosemount 4600 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 alarm direction
configuration parameter. Failure signals in multidropped transmitters are communicated
through HART messages.
Note
A minimum loop resistance of 250 ohms is required to communicate with a Field
Communicator. If a single power supply is used to power more than one Rosemount 4600,
the power supply and circuitry common to the transmitter, should not have more than
20 ohms of impedance at 1200 Hz.
A
B
C
Aotewell Ltd
www.aotewell.com
Industry Automation
HongKong|UK|China
+86-755-8660-6182