X20 system modules • Analog output modules • X20AO2438
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X20 system User's Manual 3.10
4.4.3.10.6 HART
HART (Highway Addressable Remote Transducer) is a protocol for communicating with intelligent field devices. It
was developed in order to more efficiently use the infrastructure for transferring analog signals. The digital HART
notifications are modulated to the analog signal using Frequency Shift Keying (FSK). HART can thus use the same
physical line as the analog signal without influencing the original function.
HART slaves are able to determine different process data independently and prepare HART concordantly. This
protocol supports polling of the value of a process variable as well as its unit and status. Field devices usually supply
their information after the master requests it. In newer revisions, it is also possible to transfer configuration data.
There are two different types of HART networks. In a
point-to-point
network, only one slave is connected to a HART
master. Here, the analog signal and the HART signal can be transferred over the same line. Managing several
slaves with HART requires what is known as a
multidrop
network. Here, each HART slave is assigned and identified
by a unique address. Classic analog signals cannot be clearly traced in bus systems. As a result, the HART protocol
does not support analog information transfers in multidrop networks up to and including HART Revision 5.
Information:
Split range operation with HART AO modules
Beginning with HART revision 6, bus stations that use an analog signal according to the split range
method are written to separately. The HART protocol supports multidrop addressing as well as the use
of analog signals for these applications.
The module was designed based on HART-Revision 5. Only single-channel FSK scheme is available for transmit-
ting the signals.
Since all HART frames are generated and evaluated in the application when using the FlatStream interface, infor-
mation that isn't specified until later revisions can also be read.
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