three clock pulses. Thus, whenever the VOYETRA module wants to see
the state of a key, it sends a clock pulse and the next cell is advanced.
The output of the CD4021s is routed to the CD4051 U8 address pins,
while the inputs to the CD4051 are three voltage levels: +8V, -6V and
ground. When the outputs from the CD4021s are both LOW, input 0 (or
ground) is selected. This corresponds to a key in transition. When the
outputs are LOW and HIGH, or CD4051 position 1, the +8V is selected,
signifying a key up. When the outputs are HIGH and LOW, the -6V is
selected, signifying a key down. In this way, the three possible key
positions that are encoded by two binary digits are converted to a tri-level
signal that can be sent on one line.
6.2.4 Analog Data Acquisition
When the VOYETRA module is through sensing all of the key positions,
it sends an ANALOG STROBE down the clock/power line. This is sensed
as ASTR and resets the CD4015 so all of the outputs are again LOW,
disabling the CD4021s. The CD4013 flip-flop now changes state to
disable the digital level translator (CD4051 U8) and enables the analog
multiplexer (CD4051 U9). The ASTR also sets the outputs of the CD4024
divider to a low state, addressing the first input of U9, which is the +/- Y
line.
When the VOYETRA module wants to process the next analog controller,
which in this case is the +/- X line, it sends a clock pulse which is sensed
as CK and sent to the CD4024 clock input to advance the CD4051 to the
next position. Then another clock is sent to advance it to the pressure
position. The last five positions are not used, and are meant for future
expansion.
The outputs of U8 and U9 are tied together and buffered by opamp U3
which is configured as a line driver. This data output is sensed by the
module as keyboard data and processed by the POT PANEL.
6.2.5 Keyboard Pressure Sensor
The keyboard pressure is sensed by two reflective sensors on either side
of the keyboard frame. As the keyboard is pressed, the frame moves
closer to the sensors, thus increasing the amount of reflected light. This
is converted to a voltage and processed by the LM324 opamps that allow
gain and offset to be calibrated. The user adjustment on the pressure
allows the threshold to be set so that the onset of pressure may be
adjusted.
