Instruction Manual
D103409X012
Principle of Operation
May 2013
123
Appendix A Principle of OperationAA−A
HART Communication
The HART (Highway Addressable Remote Transducer) protocol gives field devices the capability of communicating
instrument and process data digitally. This digital communication occurs over the same two‐wire loop that provides
the 4‐20 mA process control signal, without disrupting the process signal. In this way, the analog process signal, with
its faster update rate, can be used for control. At the same time, the HART protocol allows access to digital diagnostic,
maintenance, and additional process data. The protocol provides total system integration via a host device.
The HART protocol uses frequency shift keying (FSK). Two individual frequencies of 1200 and 2200 Hz are
superimposed over the 4‐20 mA current signal. These frequencies represent the digits 1 and 0 (see figure A‐1). By
superimposing a frequency signal over the 4-20 mA current, digital communication is attained. The average value of
the HART signal is zero, therefore no DC value is added to the 4‐20 mA signal. Thus, true simultaneous communication
is achieved without interrupting the process signal.
Figure A‐1. HART Frequency Shift Keying Technique
-0.5V
+0.5V
1200 Hz
“1”
2200 Hz
“0”
AVERAGE CURRENT CHANGE DURING COMMUNICATION = 0
ANALOG
SIGNAL
0
A6174
The HART protocol allows the capability of multidropping, i.e., networking several devices to a single communications
line. This process is well suited for monitoring remote applications such as pipelines, custody transfer sites, and tank
farms. See table 7‐2 for instructions on changing the printed wiring board DIP switch configuration to multidrop.
DVC6200 Digital Valve Controller
The DVC6200 digital valve controller housing contains the travel sensor, terminal box, pneumatic input and output
connections and a module base that may be easily replaced in the field without disconnecting field wiring or tubing.
The module base contains the following submodules: I/P converter, printed wiring board (pwb) assembly, and
pneumatic relay. The relay position is detected by sensing the magnet on the relay beam via a detector on the printed
wiring board. This sensor is used for the minor loop feedback (MLFB) reading. The module base can be rebuilt by
replacing the submodules. See figures A‐3 and A‐4.
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