083730300A DCN8101 Theory of Operation 161
6.4.4.5 I2C WATCH DOG CIRCUITRY
Special circuitry on the relay board monitors the activity on the I
2
C bus and drives LED
D1. Should this LED ever stay ON or OFF for 30 seconds, the watch dog circuit will
automatically shut off all valves as well as turn off the IR Source and all heaters. The
GFC Wheel motor will still be running as will the Sample Pump, which is not controlled
by the relay board.
6.4.5 MOTHERBOARD
This printed circuit assembly provides a multitude of functions including, A/D
conversion, digital input/output, PC-104 to I
2
C translation, temperature sensor signal
processing and is a pass through for the RS-232 and RS-485 signals.
6.4.5.1 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 (e.g. BOX TEMP, CO MEAS, CO REF, etc.) 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
T300/T300M is used in uni-polar mode with a +5 V full scale. The converter includes a
1% over and under-range. This allows signals from –0.05 V to +5.05 V 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.
6.4.5.2 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.
CO MEASURE AND REFERENCE
These are the primary signals that are used in the computation of the CO concentration.
They are the demodulated IR-sensor signals from the sync demodulator board.
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