4.4
OUTPUT VOLTAGE CONTROL
A key feature of the PPT is the ability to configure the pressure range to a specific application by
issuing commands from a PC or host computer. All configuration changes can be stored in the PPT so
that it will power-up with those parameters set. This allows the user to set-and-forget any custom
configuration changes and use the PPT in the analog output mode from then on. The pressure range
and analog output parameters can be tailored to a specific application by using a simple set of PPT
commands. These commands (
H=
,
L=
,
O=
, and
W=
) are described below.
The output analog voltages for the zero reading and full scale reading are set at the factory to zero
and five volts. If the analog measuring device in the application has a low level input limit of 1 volt
and a high limit of 4.85V, the PPT can be configured to accommodate this. The lower limit can be set
to any value between 0 and 5 volts in 50 millivolt increments (1% FS steps). The
AN=ON
command
must be set. For example, the low output limit can be changed from 0 volt to 1 volt by using the
command
*ddL=20
(20% x 5V = 1V). Similarly, the high output limit can be changed from 5 volt to
4.850 volt by using the command
*ddH=97
(97% x 5V = 4.85V). Figure 4.3 shows the factory-set
output levels and Figure 4.4 shows the user modified output levels configured from 1V to 4.85V
range for a 20 psia unit.
5
0
Analog Out (V)
AN=ON
AN=OFF
5
0
Analog Out (V)
AN=ON -
AN=OFF -
0
Pressure (psi)
20
20
Pressure (psi)
0
Figure 4.3—Factory Set Output Levels
The
AN
command has two options for reverse scale output:
AN=ON-
and
AN=OFF-
. When either
option is selected, the lowest pressure will output the highest analog voltage (
H=
) and the highest
pressure to output the lowest analog voltage (
L=
). Notice that the input pressure vs. output voltage
curves in Fig. 4.3 look similar. But note that the pressure values along the pressure axis have been
reversed. The curve on the right is how the analog range operates in the
AN=ON-
or
AN=OFF-
modes. When visualized in this manner, the offset (
O=
) command works similar for both curves.
That is, the offset value moves the window (
W=
) from the origin (
O=0
) to the upper right (
O=99
).
4.5
PRESSURE WINDOW CONTROL
The pressure range, or span and offset, can be user customized to accommodate a specific applica-
tion. The user can scale the PPT’s pressure range over a smaller full scale span using the
W=
com-
mand. Compressing the pressure range this way increases the output resolution per change in
pressure. The pressure range can be offset from the minimum pressure value by using the
O=
command. These commands can effectively increase the analog output resolution by spreading it
over a smaller pressure range than full scale.
The O= command offsets the pressure window to reassign the minimum pressure value for the
lowest analog output. This command sets a pressure offset value from 0 to 99% of full scale. The
default is O=0%FS.
The W= command sets the width, or span, of the pressure window to a smaller range to fill the
analog output limits. This command sets a pressure window value, or span, from 0 to 99% of full
scale. A W=0 command will set the window to 100% and is the default value. For our example above,
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