Chapter 8 Theory of Operation
Temperature Control Board
Two 50 watt 100 ohm resistors (400 ohm for 220 V) mounted on the optical bench are
used to heat the optical bench above the dew point, to avoid moisture condensation on the
mirrors. A thermistor is used to determine the bench temperature, with op-amp U1 and
the solid state relay K1 used as the control elements to control the current into the
heaters.
The Microprocessor System
The microprocessor system consists of P.C. boards which plug into a motherboard,
connecting them to each other and to the rest of the instrument. These boards are as
follows:
Display Module
Processor Board
Analog to Digital Board
Digital to Analog Board
C-Link Board
Display Module.
The vacuum fluorescent display module shows CO concentrations,
instrument parameters, and help messages. The single board display module consists of
80 characters (4 line by 20 column), refresh memory, character generator, dc/dc converter
and all necessary control logic. The display module is powered by +5 volts dc.
Processor Board.
The Processor Board contains a Motorola M68HC11F1
microprocessor (U14), RAM (U5), and EEPROM (U2). In addition, this high-
performance, nonmultiplexed 68-pin microprocessor, contains 512 bytes of EEPROM
and 1K of RAM. It is operated at a frequency of 1 MHz, which is generated by crystal
X1.
During each instruction cycle, the processor fetches an instruction from memory and
executes it, reading or writing data to or from the data bus, or performing a calculation on
some internal register or registers. The reset signal is generated by U7. This signal
resets the M68HC11F1 every tenth of a second, and is used by the microprocessor to
keep track of time. Each time the microprocessor is reset, it reads the counters,
increments the clock, checks the status of the pushbuttons, and updates the D/A
converters and display. The MC6840 counter chip (U1) acts as the interface between the
Input Signal Conditioning Board and the microprocessor. A pulse train from the Input
Signal Conditioning Board is directly fed into one of three counters on the MC6840
counter chip.
8-3
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