Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 225)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 207
STATUS LED’s
DC VOLTAGE TEST
POINTS
RELAY PCA
Figure 9-15: Location of relay board Status LED’s
9.3.5.6. 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.
9.3.6. 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.
9.3.6.1. 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.