Hart Mode Communication; Figure 5 - Hart Point-To-Point And Multi-Drop Value Scaling - Honeywell ST 800 Series User Manual

2.3 HART Mode Communication

When using MCT202,before connecting to a HART transmitter, verify that the FDC
application is used and not the MC Toolkit application. When you use the MC Toolkit
application, the MCT202 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.
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Transmitters with HART capability have features that vary among manufacturers and with
the characteristics of specific devices. The FDC software application executing on the
MCT202 supports the HART Universal, Common Practice and Device Specific Commands
which are implemented in the Honeywell Transmitters.
As indicated in Figure 5, the output of a Transmitter configured for HART protocol includes two
primary modes:
Figure 5 – HART Point-to-Point and Multi-drop Value Scaling
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Point-to-Point Mode, in which one Transmitter is connected via a two-conductor, 4-20 mA
current loop to one receiver.
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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, almost identical to that of a Transmitter operating in analog mode. 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 level 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.
Revision 5.0 ST 800 Series HART/DE Option User's Manual Page 7