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6.3.2 M
OTHERBOARD
This printed circuit assembly provides a multitude of functions including, A/D
conversion, digital input/output, PC-104 to I
processing and is a pass through for the RS-232 and RS-485 signals.
6.3.2.1
A TO D CONVERSION
Analog signals, such as the voltages received from the analyzer's various sensors, are
converted into digital signals that the CPU can understand and manipulate by the analog
to digital converter (A/D). Under the control of the CPU, this functional block selects a
particular signal input and then converts the selected voltage into a digital word.
The A/D consists of a Voltage-to-Frequency (V-F) converter, a Programmable Logic
Device (PLD), three multiplexers, several amplifiers and some other associated devices.
The V-F converter produces a frequency proportional to its input voltage. The PLD
counts the output of the V-F during a specified time period, and sends the result of that
count, in the form of a binary number, to the CPU.
The A/D can be configured for several different input modes and ranges, but in the T400
it is used in uni-polar mode with a +5V full scale. The converter includes a 1% over and
under-range. This allows signals from –0.05V to +5.05V to be fully converted.
For calibration purposes, two reference voltages are supplied to the A/D converter:
Reference Ground and +4.096 VDC. During calibration, the device measures these two
voltages, outputs their digital equivalent to the CPU. The CPU uses these values to
compute the converter's offset and slope and uses these factors for subsequent
conversions.
6.3.2.2
SENSOR INPUTS
The key analog sensor signals are coupled to the A/D through the master multiplexer
from two connectors on the motherboard. 100K terminating resistors on each of the
inputs prevent cross talk from appearing on the sensor signals.
O
D
ETECTOR
3
This is the primary signal used in the computation of the O
G
P
AS
RESSURE
This sensor measures the gas pressure in the sample chamber upstream of the critical
flow orifice. The sample pressure is used by the CPU to calculate O
G
F
AS
LOW
This sensor measures the flow rate of the sample gas through the instrument. This
information is used as a diagnostic tool for determining gas flow problems.
6.3.2.3
THERMISTOR INTERFACE
This circuit provides excitation, termination and signal selection for several negative-
coefficient, thermistor temperature sensors located inside the analyzer. They are as
follows:
083730400A DCN8130
O
UTPUT
S
ENSOR
S
ENSOR
Teledyne API – Model T400 O
2
C translation, temperature sensor signal
concentration.
3
Concentration.
3
Analyzer
3
149
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