Radio Shack TRS-80 Service Manual page 49

Color Computer 2 Service Manual
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Next, a value equivalent to the midpoint of the voltage
range, 32 decimal for example, is sent to the DAC's six-bit
input via PIA U2. If the DAC output voltage is lower than
the joystick voltage, the HI/LO pin of the DAC chip
(comparator output) is low. However, if the DAC voltage is
higher, the HI/LO pin is high (logic one). If the HI/LO pin
was low, the next value the microprocessor tries is half-way
between 0 and the last value, or 16 decimal. If the HI/LO
pin was high, the microprocessor tries a value half-way
between the last value and 64, or 48 decimal. For each
value, the microprocessor either adds or subtracts half the
value of the preceding difference, until the final value is
found. Since the DAC uses six binary digits, exactly six
DAC settings are required to determine the "position" along
one axis of the joystick. The four joystick inputs to the
DAC chip correspond to two axes for each of two joystick
controllers.
The sound multiplexing section is very simple. According to
the above table, different sound sources are selected by the
Sel A and Sel B inputs, and the selected input is routed to
the sound output. If the DAC is used as a sound source, the
microprocessor simply feeds a succession of values to the
six bits of the DAC in order to produce the desired
waveshape. The output of the DAC is then buffered and
attenuated to provide approximately 3.9 volts p-p, which is
the level required by the modulator to produce maximum
volume. If the cassette is the selected input, then sound
from the cassette recorder is routed to the sound output.
This level follows the input level up to 3.9 volts p-p, at
which point it clips the input waveform. Therefore, the
volume control on the cassette should not be set higher than
the level which provides 3.9 volts p-p to the DAC chip.
Similarly, the cartridge may supply the sound source (from
pin 8 of the cartridge connector). This signal may be
AC-coupled, since the SND input to the DAC chip biases the
input at the midpoint of the allowable voltage swing, which
is 3.9 volts p-p. Any greater signal amplitude will result
in clipping (distortion) of the sound waveform.
In addition to the Select inputs, the sound must be enabled
by bringing SNDEN to a high level. This input is controlled
by PIA U2. If this pin is at a low level, all sound (except
single bit sound) is disabled.
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