Teledyne 6200E - Sulfides Analyzer - Manual (Page 228)

Theory Of Operation Model 6200E Instruction Manual
228 M6200E Rev: A1
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.
10.4.10. 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.
10.4.10.1. 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 in the 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 A to D 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
and outputs their digital equivalent to the CPU. The CPU uses these values to compute the A to D
converter’s offset and slope (not the same offset and slope recorded during zero/span calibration)
and uses these factors for subsequent conversions. See Section 6.9.4.6 for instructions on
performing this calibration.
10.4.10.2. 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 H
2
S, H
2
S and H
2
S concentrations displayed at the top right hand corner of the front panel
display and output through the instruments 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 (Section 6.2.1) through
the analyzer’s front panel.