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XNX Universal Transmitter
Appendix A - HART
®
Protocol
167
A.1 HART
®
Interface
The XNX Universal Transmitter is registered with the
HART Communication Foundation.
Note: Only qualified service personel should perform the procedures in this section.
Every XNX
®
gas sensor can communicate using the HART protocol
(dened by the HART Communication Foundation at http://www.
hartcomm.org). HART is unique among eldbuses in that the digital
signal is superimposed on a traditional 4-20 mA current loop. This
provides the reliability of analog signaling with the advanced
diagnostic capability of a digital device.
HART devices are usually connected as point-to-point networks. The
analog output of the XNX transmitter can also be disabled to facilitate
construction of multidrop, all-digital HART networks.
Control System
or Other Host Application
Multiplexer
Barrier
Handheld Terminal
Field Device
Note: Instrument power is provided by an interface
or external power source that is not shown.
Figure 233. HART Point-to-point Mode
If HART is not needed, the unit can be used as a 4-20 mA transmitter.
Since the transmitter is a slave, the internal modem will remain silent
if no master signal is present. Additionally the HART signal is at too
high a frequency (1200 Hz) to interfere with analog control equipment.
Another novel feature of HART networks is that two masters can
be present. The primary master is usually a distributed control
system (DCS), programmable logic controller (PLC), or a personal
computer (PC). The secondary master can be a handheld terminal.
The XNX transmitter has been tested with the handheld Emerson eld
communicator.
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 234. HART Multi-point Mode
The XNX device descriptor (DD) le provides HART users with data
on the capabilities and features of the XNX Universal Transmitter.
Select HART enabled devices are able to interface with XNX
transmitters when connected via HART communication. A copy of
the le is included on the Documentation CD. This DD le can be
installed on HART-enabled Emerson eld communicators using the
Emerson Easy Upgrade Utility. The DD les located on the resource
CD are compatible with the software integral to the transmitter. Older
transmitters using earlier versions of software require previous versions
of the DD les. Contact your local Honeywell representative with any
questions regarding software compatibility.
During manufacturing, Honeywell congures the 8-digit HART tag to
the XNX serial number. This can be used to conrm correct wiring
from the transmitter to the control system. If desired, the HART tag
can be modied. The xed XNX serial number can also be read over
HART.
For convenience, the transmitter presents the HART signal on two
interfaces. The 1200 Hz AC signal is capacitively coupled to the main
20 mA analog output. This may be monitored at the control system
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