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2.2
Description of the HART communication
1
The HART
protocol is supported by many conventional 4 mA ... 20 mA field devices, which thus enable
digital communication for configuration and servicing purposes. Many device parameters and also the mea-
sured values themselves can thus be digitally transferred to and from the device. This digital communication
runs in parallel with th e 4m A. . .2 0mA signal on the same cable. This is possible through a current modu-
lation, which is superimposed on the user signal.
20 mA
Analogue
signal
4 mA
Figure 2.3: The modulated HART-Signal
HART is a master-slave protocol: a field device does only respond when requested (except in "Burst mo-
de"). The message duration is several hundred milliseconds, so that between two and three messages can
be transferred per second.
On HART, there are three groups of commands:
•
The "Universal" commands; these must be supported by all field devices;
•
the "Common practice" commands; these are pre-defined commands, suitable for many field devices,
which, if they are supported by the device, must be implemented in the pre-defined form;
•
device-specific commands; these are commands, which are particularly suitable for this field device.
The HART multiplexer contains commands in all three groups. Details of the supported commands are
given in section 6.1.
1. HART = Highway Addressable Remote Transducer
Subject to reasonable modifications due to technical advances.
Pepperl+Fuchs Group • Tel.: Germany (06 21) 7 76-0 • USA (330) 4 25 35 55 • Singapore 7 79 90 91 • Internet http://www.pepperl-fuchs.com
FSK signal
+0,5 mA
0
-0,5 mA
1200 Hz
2200 Hz
"0"
"1"
R
C
C = Command
R = Response
HART Multiplexer Master KFD2-HMM-16
R
C
The high frequency HART signal consists of
the sinusoidal frequencies 1200 Hz and
2200 Hz. This signal has an average value of
zero, so that it does not affect the analogue
signal. It is removed by standard analogue
input circuit filtering.
Time (seconds)
Copyright Pepperl+Fuchs, Printed in Germany
Product description
C
R
15
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