Teledyne GFC-7000E - Trace CO2 Analyzer - Manual (Page 181)

Model GFC7000E Instruction Manual THEORY OF OPERATION
04584 Rev A1 170
STATUS LED’s
DC VOLTAGE TEST
POINTS
RELAY PCA
PN 04135
Figure 10-14: Location of relay board Status LED’s
10.3.5.2. I 2 C 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 watchdog circuit will automatically shut
of 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.
10.3.6. Mother Board
This printed circuit assembly provides a multitude of functions including, A/D conversion, digital
input/output, PC-104 to I2C translation, temperature sensor signal processing and is a pass
through for the RS-232 and RS-485 signals.
10.3.6.1. 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, CO2 MEAS, CO2 REF, etc.) and then coverts 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 MGFC7000E 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.