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1.1.
T
HE
HART
S
YSTEM
The
HART
(
H
ighway
A
ddressable
R
emote
T
ransducer – bus addressable field devices) is a digital communication protocol developed by ROSEMOUNT for industrial
measurement applications. A digital signal of small amplitude is placed on the widely used standard 4…20mA output current, and due to its symmetric sinusoidal
nature and its small amplitude, it does not affect the accuracy of the output current. Since no extra cable is needed for signal transfer, the already existing cable can
be used to realize the transfer. The identification, programming and querying of the transmitters can be done using this communication.
This diagram shows the output signal of a
4…20mA transmitter (device) and the
superimposed HART signal, which is essentially a
0.5 mA modulation of the output current..
t
I
out
[mA]
20 mA
4 mA
1
I
o
+ 0.5 mA
I
o
I
o
– 0.5 mA
0
The enlarged picture shows that HART
communication uses frequency modulation, and
logical „1” is assigned to 1200 Hz, while „0” is
assigned to 2200 Hz. This is called FSK
(Frequency-Shift Keying) modulation. The
communication is realized with systematic series
of impulses above described.
1.1.1.
The Communication
The HART communication is basically a master-slave type, which means that the transmitter – slave – only sends a response when the master (of which there can be
only one in the system) sends a query. The master can be a Hand-held Communicator, a PC with a HART modem or a universal interface, in this case, a MultiCONT
(naturally, only one can be active). The communication uses standard commands (see chapter 1.1.3)
1.1.2.
The structure of the system
The system must contain a 230..1000
Ω
resistor to guarantee that the HART signal of small amplitude will not load the power supply of unknown output impedance.
Summary of Contents for MultiCONT
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