Technical Manual
19
Liquid Crystal Display
The Model 767 uses a liquid crystal display to display data to the user. Three communication lines
and 18 segment lines connect the LCD to the display driver (U1-60 to U1-77) internal to the NEC
microprocessor. The LCD is 3:1 multiplexed with 1/2 bias. The bias voltages (1.5 volts, and 3.0 volts)
are supplied to the display driver by the voltage reference circuit
The LCD glass is electrically tied to the display PCB via an elastomeric connector sandwiched and
compressed between the glass and the PCB by 8 hex screws.
Backlight (Optional)
The backlight is an Electroluminescent style flat panel lamp residing directly under the LCD.
An oscillator is required to convert battery DC voltage to an AC voltage of about 80 volts and 300 Hz.
Whenever anything is being displayed, the microcontroller brings U1-42 low. U1-42 connects to the
backlight oscillator on the Display PCA via J2-22. VDD is also supplied to the oscillator via J2-24.
The oscillator consists of Q101, Q102, Q108, Q109, C102, R106, R107, R108, R109, D101, T1, and
the lamp itself (DS2). C106 and C107 are not populated.
Horn
The horn is activated at the start of a temperature taking cycle, at the end of a Normal mode
temperature cycle, and for various error conditions.
· A short duration single beep is indicative of normal operation.
· A short duration double beep is used to indicate errors and warnings such as
switching from Normal mode to Monitor mode during a temperature cycle.
The horn is a piezoelectric ceramic resonator driven by the processor square wave. The horn control
signal comes from U1 pin 49. It directly drives the horn LS1.
Probe Switch
When the probe has been inserted in the probe holder, the probe switch (S2) brings PROBE_SW (U1-
15) high (VCC). When the probe is removed from the probe holder, the probe switch brings PROBE
SW (U1-15) low (GND).
Serial Communications Port
Transmit , receive data and control CMOS-level signals are made available for test or system
integration via J3. The contacts of J3 are laid out so they are accessible through a slot in the rear of
the case. Communications on a cable length of over one foot should be driven with external RS232 or
other line driving circuitry.
Mode Button
When momentarily (less than 2 seconds) depressing the Mode button (S1), U1-26 goes low. The
microcontroller then toggles the display from F to C or C to F. When the instrument is asleep (no
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