DESCRIPTION AND OPERATION
FUNCTIONAL OPERATION
2 - 4
WBPEEUI110506A0
seconds. This causes the electronics to automatically adjust
the output to 100 percent. Span or zero or both may be
adjusted.
Communications
A high frequency AC voltage signal imposed on the signal
wires, known as FSK (frequency shift keying), allows commu-
nications between the Type PTSP pressure transmitter and the
Type STT terminal. The location of the Type PTSP pressure
transmitter may be up to 1.6 kilometers (1.0 mile) from the
Type STT terminal. The FSK method provides excellent noise
immunity for remote communications. A minimum of
250-ohms loop resistance is necessary to support communica-
tions (Fig.
). This is true whether the transmitter is config-
ured in the analog (four to 20 milliampere) point-to-point mode
or the digital field bus mode. The Type STT terminal connects
to the target transmitter anywhere there is access to the signal
leads of the transmitter (for point-to-point wiring, the Type
STT terminal must be connected between the device and the
250-ohm resistance). The clip leads connect across the signal
leads independent of signal direction or polarity. Since the
communication signal, a high frequency AC, has a zero DC
average, it has no effect on the output. Two different frequency
levels transmit a logic zero or a logic one.
Configuring the transmitter in the digital mode (refer to
) causes the microcontroller to set the output of the
transmitter to less than four milliamperes for low power con-
sumption. The transmitter then provides a digital process vari-
able signal. The transmitter reports its output (in percent) up
to ten times per second for control purposes (Fig.
).
NOTE: Refer to the
Field Bus I/O Module (IMFBS01) Instruction
for wiring practices and output update times.
The FSK communication technique makes it possible to com-
municate in the presence of electrical noise. Figure
shows
how the ability to communicate varies with the amplitude and
frequency of the noise. This graph represents a typical instal-
lation and applies only when one noise frequency is present.
The graph indicates that the FSK communication technique
tolerates up to two volts of noise amplitude at 60 hertz. This
tolerance decreases as the noise frequency approaches the
FSK communication frequencies of 19.0 kilohertz and
30.5 kilohertz.
Temperature Compensation
The transmitter electronics monitor the temperature of the cell
assembly. This is accomplished by monitoring the resistance
of one of the legs of the Wheatstone bridge within the cell. This
technique provides a true cell temperature measurement, and
Содержание platinum standard PTSP Series
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