XNX Universal Transmitter
Installation and Operation
57
Multidrop Mode
The multidrop mode of operation requires only a single pair of
wires and, if applicable, safety barriers and an auxiliary power
supply for up to 8 eld devices (see the illustration below). All
process values are transmitted via HART protocol. In multidrop
mode, all eld device polling addresses are >0, and the current
through each device is xed at a minimum value (typically 4 mA).
Input/Output (I/O) System
Handheld Terminal
Field Devices
Control System or
Other Host Application
Note: Instrument power is provided by an interface
or external power source that is not shown.
Figure 56. Multidrop mode of operation
Note: Use multidrop connection for supervisory control installations that are widely
spaced such as pipelines, custody transfer stations, and tank farms.
!
In general, the installation practice for HART devices is the same
as conventional 4-20mA instrumentation. Individually shielded
twisted pair cable, either in single-pair or multi-pair varieties, is
the recommended wiring practice. Unshielded cables may be
used for short distances if ambient noise and cross-talk will not
affect communication.
The minimum conductor size is 0.51 mm diameter (#24 AWG) for
cable runs of less than 5,000 ft. (1,524 m) and 0.81 mm diameter
(#20 AWG) for longer distances.
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