Fisher FIELDVUE DVC6200 Instruction Manual page 96

DVC6200 Digital Valve Controller
+0.5V
0
−0.5V
1200 Hz
"1"
AVERAGE CURRENT CHANGE DURING COMMUNICATION = 0
A6174/IL
Table A-1. HART Frequency Shift Keying Technique
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
A
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.
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
88
ANALOG
SIGNAL
2200 Hz
"0"
CONTROL SYSTEM
HART
MODEM
FIELD
TERM.
E1362
Figure A-2. Typical FIELDVUE Instrument to Personal Computer
Connections for ValveLink Software
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.
DVC6200 digital valve controllers are loop-powered
instruments that provide a control valve position
proportional to an input signal from the control room.
The following describes a double-acting digital valve
controller mounted on a piston actuator.
The input signal is routed into the terminal box through
a single twisted pair of wires and then to the printed
wiring board assembly submodule where it is read by
the microprocessor, processed by a digital algorithm,
and converted into an analog I/P drive signal.
As the input signal increases, the drive signal to the
I/P converter increases, increasing the I/P output
pressure. The I/P output pressure is routed to the
pneumatic relay submodule. The relay is also
connected to supply pressure and amplifies the small
pneumatic signal from the I/P converter. The relay
accepts the amplified pneumatic signal and provides
two output pressures. With increasing input (4 to 20
mA signal), the output A pressure always increases
September 2010