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Model 360E Instruction Manual THEORY OF OPERATION
10.3.2. CPU
The Model 360E’s CPU is a, low power (5 VDC, 0.8A max), high performance, 386-based microcomputer
running MS-DOS. Its operation and assembly conform to the PC/104 Specification version 2.3 for embedded
PC and PC/AT applications. It has 2 MB of DRAM on board and operates at 40MHz over an internal 32-bit data
and address bus. Chip to chip data handling is performed by two 4-channel DMA devices over data busses of
either 8-bit or 16-bit configuration. The CPU supports both RS-232 and RS-485 serial I/O.
The CPU includes two types of non-volatile data storage.
Disk-On-Chip
While technically an EEPROM, the Disk –on-Chip (DOC), this device appears to the CPU as, behaves as, and
performs the same function in the system as an 8MB disk drive. It is used to store the operating system for the
computer, the Teledyne Instruments Firmware, and most of the operational data generated by the analyzer’s
internal data acquisition system (iDAS - See Section 6.7).
Flash Chip
Another, smaller EEPROM used to store critical calibration and configuration data. Segregating this data on a
separate, less heavily accessed chip significantly decreases the chance of this key data being corrupted.
10.3.3. Optical Bench & GFC Wheel
Electronically, the Model 360E’s optical bench, GFC wheel and associated components do more than simply
measure the amount of CO
2
present in the sample chamber. A variety of other critical functions are performed
here as well.
10.3.3.1. Temperature Control
Because the temperature of a gas affects its density and therefore the amount of light absorbed by that gas it is
important to reduce the effect of fluctuations in ambient temperature on the Model 360E’s measurement of CO
2
.
To accomplish this both the temperature of the sample chamber and the GFC Wheel are maintained at constant
temperatures above their normal operating ranges.
Bench Temperature:
To minimize the effects of ambient temperature variations on the sample measurement, the
sample chamber is heated to 48C (8 degrees above the maximum suggested ambient operating temperature
for the analyzer). A strip heater attached to the underside of the chamber housing is the heat source. The
temperature of the sample chamber is sensed by a thermistor, also attached to the sample chamber housing.
Wheel Temperature
: To minimize the effects of temperature variations caused by the near proximity of the IR
Source to the GFC wheel on the gases contained in the wheel, it is also raised to a high temperature level.
Because the IR Source itself is very hot, the set point for this heat circuit is 68C. A cartridge heater implanted
into the heat sync on the motor is the heat source. The temperature of the wheel/motor assembly is sensed by a
thermistor also inserted into the heat sync.
Both heaters operate off of the AC line voltage supplied to the instrument.
10.3.3.2. IR Source
The light used to detect CO
2
in the sample chamber is generated by an element heated to approximately 1100
o
C
producing infrared radiation across a broad band. This radiation is optically filtered after it has passed through
the GFC Wheel and the sample chamber and just before it reaches the photo-detector to eliminate all black body
radiation and other extraneous IR emitted by the various components of those components.
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