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STT700 Series HART/DE Option User’s Manual
Revision 1
2.3 HART Mode Communication
When using MCT404, but before connecting to a HART transmitter, verify that the FDC
application is used and not the MC Toolkit application. When using the MC Toolkit application,
the MCT404 is set for DE communications, where the current amplitude can
bump
process
variables in either point-to-point or in the multi-drop mode in HART.
Transmitters with HART capability have features that vary among manufacturers and
with the characteristics of specific devices. The FDC software application executing on
the MCT404 supports the HART Universal, Common Practice and Device Specific
Commands which are implemented in the Honeywell transmitters.
As indicated in Figure 6, the output of a transmitter configured for HART protocol includes two
primary modes:
Figure 6
– HART Point-to-Point and Multi-drop Value Scaling
Point-to-Point Mode, in which one transmitter is connected via a two-conductor, 4-20 mA
current loop to one receiver.
Multi-Drop Mode, in which several transmitters are connected through a two-conductor
network to a multiplexed receiver device.
In point-to-point mode, the value of the primary Process Variable (PV) is represented by a 4-20 mA
current loop, and is almost identical to that of a transmitter operating in analog mode. Additionally,
one device can reside in analog output mode when it is configured in multi-drop. In this case, however,
the analog signal is modulated by Frequency Shift Keying (FSK), using frequencies and current
amplitude that do not affect analog sensing at the receiver. The accuracy of the analog value must be
precisely controlled for accurate sensing. HART communication will not
bump
process variables.
In multi-drop mode, up to 16 transmitters in HART 5 (addresses 0-15) and up to 64 transmitters in
HART6/7 (addresses 0-63) can exist on the two-conductor network.
