Welch Allyn GSI 38, Service Manual

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The Tymp Microp rocessor controls the following functions:
Communication to the Audio Microprocessor ; Asynchronous communications data is
transmitted and received using the processors built in serial communications interface.
The interface is located within port D of the microprocessor. PDO is the Tymp receive data
line (TRXD) and PD1 is the Tymp transmit data line (TTXD). The bidirectional
communication allows the transfer of information, test results, parameters, and control
functions to occur between the two processors.
Pump Drive Control: The Pump assembly consists of a DC step motor, an air reservoir,
a piston, and an infra photo eye/detector pair. After power-up (during the instrument
initialization period) and at the end of a test the pump piston is returned to the home
position. The home position (HOME) is determined by a state transition of the infra red
photo detector of the pump assembly which is monitored by the Tymp microprocessor
(U7) A/D input PE4 (pin 44). The pump motor coils are driven by the motor driver (U37)
which is clocked by the pump drive timer (U21). The timer controls the step rate of the
pump assembly which varies with the pressure sweep rate. The pump sweeps at a rate
of 600 daPa/sec until the slope of the tympanogram is sensed then the sweep rate slows
to 200 daPa/sec. If a leak is detected during the pump sweep the pump stops until the
probe is removed from the ear canal. When the probe is removed from the ear canal the
pump returns to the home position.
Monitoring of the Pressure Transducer Output The pressure Transducer output is
monitored by the Tymp Microprocessor (U7) A/D input PE6 (pin 48). In the Ambient
Pressure calibration mode the CMOS Switch (U64) is used to adjust the gain of the
pressure transducer circuit so that the input voltage at the A/D is equal to 1.5 vdc. This
gain adjustment places the pressure transducer output at ambient or zero pressure in the
optimum operating range for the A/D convertor. When the ambient gain has been
established the processor will set the switches to the stored values at all times. The
pressure span calibration (+200/-400) is controlled by the technician and is stored by the
microprocessor as a software offset.
Probe Tone Oscillator (226 Hz) and Speaker Drive
The Probe tone Frequency is
generated by the Microprocessor (U7) Internal Timer System (Port A). This produces a
226 Hz Square wave at PA3 of the Tymp Microprocessor (U7 pin 31). The Square wave
level or gain is determined by the CMOS switch settings of (U45 and (U46). These
switches select the appropriate output levels based on controls from the microprocessor.
The output levels or switch settings are determined by the calibration data and the external
volume as measured by the probe microphone. The appropriate level is then routed
through a 226 Hz band pass filter (U61). The signal is then routed to the Probe Tone / lpsi
speaker amp (U66).
Microphone Input level Monitoring The microphone signals that are measured in the
ear canal must be filtered to remove as much unwanted signal as possible. This filtering