090-252 Rev A
3-1
3 SIGNAL CONDITIONING CARD USER INTERFACE
Plugging the HHPC into the Signal Conditioning Card allows the user access to some
quantities and features that cannot be accessed by plugging in to the CPU Card (the
RJ11 jack on the left of the Interconnect Card). The Signal Conditioning Card can only
be accessed by plugging into the RJ11 jack on the right side of the Interconnect Card.
It cannot be accessed remotely from the control room.
3.1 Continuous Display Mode
When the HHPC boots up while connected to the Signal Conditioning Card, it is put
first into the continuous display mode for the Signal Conditioning Card. The user is
given continuously updated information. The first items are the resistance of the
temperature sensor and the actual A/D counts read from it, then the frequency and
period of the density signal along with the actual number of timer counts between
each reading. Next are the lower and upper level sensor voltages and the backup
A/D reading.
The voltage from the temperature sensor in the probe is input to the main A/D
converter on the Signal Conditioning Card. It is also input to a less accurate 8-bit
A/D converter on the microprocessor on the Signal Conditioning Card. This provides
a secondary reading to the main A/D converter. In the event that the main A/D
converter is suspected to be defective, the secondary A/D converter can give an
approximate voltage reading.
Note that the result from the main A/D is reported as resistance, and the result from
the secondary A/D is reported in voltage. To calculate the approximate value of the
voltage reported by the main A/D using the reported resistance, multiply by 0.001,
which is the approximate value of the excitation current of the temperature sensor.
To gain a general idea of the conditions inside the Control Unit, a temperature
sensing chip was placed on the Signal Conditioning Card. This temperature is
reported in Fahrenheit and Celsius for convenience. Note that the temperature
reported by the chip is actually slightly higher than the ambient temperature in the
box due to self-heating of the chip.
Finally, there are several status bits from the A/D converter and the level sensor
diagnostics that are reported if there is an error condition. They are as follows:
OV – A/D Converter Overflow
OD – A/D Converter Oscillation Detect
LS – Lower Level Sensor Shorted
LO – Lower Level Sensor Open
UO – Upper Level Sensor Open
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