Teledyne 6200T - Sulfides Analyzer - Manual (Page 315)

6200T Total Sulfur Analyzer Principles of Operation
Teledyen Analytical Instruments 315
As a safety measure, special circuitry on the relay board watches the status of LED D1.
Should this LED ever stay ON or OFF for 30 seconds, indicating that the CPU or I
2 C
bus has stopped functioning, the Watchdog Circuit will automatically shut of all valves
as well as turn off the UV Source(s) and all heaters. The Sample pump will still be
running.
13.5.8. 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.
13.5.8.1. 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 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 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. Refer to Section 5.9.3.6 for instructions on performing this calibration.
13.5.8.2. 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.
PMT DETECTOR OUTPUT: This signal, output by the PMT preamp PCA, is used in
the computation of the SO
2
, CO
2
and O
2
concentrations displayed in the front panel
display screen and output through the instrument’s analog outputs and COMM ports.
PMT HIGH VOLTAGE POWER SUPPLY LEVEL: This input is based on the drive
voltage output by the PMT pram board to the PMT’s high voltage power supply
(HVPS). It is digitized and sent to the CPU where it is used to calculate the voltage
setting of the HVPS and stored in the instruments memory as the test function HVPS.
HVPS is viewable as a test function (refer to Section 4.1.1) through the analyzer’s front
panel.
PMT TEMPERATURE: This signal is the output of the thermistor attached to the
PMT cold block amplified by the PMT temperature feedback circuit on the PMT preamp
board. It is digitized and sent to the CPU where it is used to calculate the current
temperature of the PMT.