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149-072011 Model 2002 Vacuum Gauge Page 14 of 38
4. Rear Panel Description
A schematic of the rear panel of the Model 2002 control unit is shown in Figure 4.2 ( Rear Panel Figure).
The transducer is connected to the control unit via an female 9-pin “D” connector on the rear panel. An I/O
cable can be connected via a male 9-pin “D” connector on the rear panel. The I/O cable provides an analog
output signal, a remote zero input capability, high and low alarm TTL outputs, and a + 5 VDC Power
Supply Output. The I/O cable also provides an analog ground and a digital common.
4.1. Remote Zero Input
The Model 2002 can be zeroed remotely via the I/O cable using the remote zero input line. Upon setting this
line to TTL low level, the instrument will set its current pressure reading to zero. A common application of
this feature is to utilize a process control output from a high vacuum gauge (for example an ion gauge
controller) to automatically zero the Model 2002 whenever the pressure is below the low 10-5 Torr range. A
Digital Common is provided on pin # 5 which can be used with relay contacts or switch contacts to remotely
zero the instrument.
4.2. Analog Output
The Model 2002 provides an analog voltage output signal (0.5 VDC linear per decade) for process control
and for pressure monitoring. A plot of the analog output signal as a function of pressure is shown in Figure
4.1 . Note that the output voltage is directly proportional to the pressure over each complete decade (see
detailed region in Figure 4.1) and that each decade has a different linear slope.
If the analog output voltage is being used in a data acquisition system, the following formulas can be used to
calculate pressure.
Above an indicated pressure of 103 Torr, the analog output signal will be +5 VDC. Below the minimal
displayable pressure of 1 x 10-4 Torr, the analog output will be 1.0 VDC.
18
)1(
2
)6(
−
+
+
=
MANTEXP
V
The analog output voltage is given by the following equation:
Where EXP is equal to the exponent of the displayed pressure.
EXP=-2
MANT=1.53
(
)
(
)
[]
VDC
V
0294.2
294.02
18
153.1
2
62
=
+=
−
+
+
−
=
Example: P=1.53x10
-2
EXP=-2
MANT=9.87
(
)
(
)
[]
VDC
V
4928.4
4928.04
18
187.9
2
62
=
+=
−
+
+
−
=