Kurzweil K2000 Service Manual page 95

SECTION 5
AUDPWR1.00c 12/22/92
contains EPROM (to hold a monitor program), RAM (to hold the program being debugged), serial port
logic to talk with the development host, and logic to recreate ports 0 - 2.
NOTE: This port is normally only used at depot level service. Do not attempt to use this port
for debugging unless specifically instructed. The purpose for this discussion was to define the
reason for the extra pins at J3 and the need for JPl to be intact for normal operation.
5.1.1.4 FRONT PANEL INTERFACE
The K2000 front panel consists of 38 pushbuttons, 8 LEDs, and a continuous rotation "spin knob".
The buttons are wired into a 10 column by 4 row matrix. U31 (B-4 to B-S), receiving 4-bit binary
information from the M374S0 drives one column at a time low. The 4 rows are held high by pull-up
resistors in RP9 (E-2). If a button is pressed, the appropriate row is pulled low and sensed by
the M374S0. Because there are no isolation diodes for the front panel, the number of simultane-
ous button closures that can be sensed is limited to 2 unless they are all in the same row or
column.
The LEDs are wired in an 8 column by 1 row matrix, driven by a transistor current booster Ql (B-
4). To turn an LED on, the M374S0 will activate Ql by driving Port 6, bit 7 high when the column
connected to the LED is selected. Ql is normally off when the M374S0 is actually sensing buttons
in order to minimize the saturation voltage of U31 and give a good logic low level to sense. Ql
is an emitter-follower which minimizes current drawn from the M374S0. R81 suppresses possible
emitter-follower oscillations.
The spin knob is a rotary switch with 2 contacts, 36 pOSitions, and continuous rotation. Each
contact is either open or connected to GND2. The switching sequence (Gray code) is such that the
M374S0 program can detect which direction the switch is being rotated and how many positions.
One property of this code is that if the two contacts connections are reversed, the direction of
rotation will interpreted as reversed by the M374S0.
U60 (C-S) is used to interrupt the Engine microprocessor by activating EXTIRQb at pin 6. This is
accomplished by taking column C high while column D is already high. The Engine acknowledges
this, and resets U60, by sending EXTIAKb to pin 1 of the flip-flop.
5.1.1.5 WHEELS INTERFACE
The wheels are standard rotary potentiometers actuated by large "thumbwheel" knobs. Only 1/3 of
the allowable rotation is used, thus the mechanical center must correspond fairly close to the
electrical center of the pots. Amplifiers US9-7 and U59-8 provide the necessary gain so that 1/3
rotation results in a full 0 to 5 volt swing for the M374S0's A/D inputs. C194 and C191 filter
noise and smooth the output. R149, R1SO, DS and D6 prevent overload of the M374S0
AID
inputs in
the event of gross wheel mis-adjustment.
The Wheels circuit board itself contains a network for each wheel which allows electrical ad-
justment of the centering and exact amount of amplifier gain. Figure 5.1.1.2 below is the
schematic for one wheel; both circuits are identical. The circuit board's pots are adjusted to
match the characteristics of each particular wheel's pot.
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1
Figure 5.1.1.2 - Wheel adjustment circuit
5.1.1.6 PRESSURE SENSOR INTERFACE
The keyboard pressure sensor is a long strip between the keys and the keybed. When pressure
greater than that needed to hold a key down is applied, the strip is compressed and changes from
an open circuit to a resistance that varies with the amount of pressure.
R206 and D19 together make a 0.6 volt source with just a few ohms of dynamic impedance resulting
in a reasonably linear current/pressure curve. This voltage is applied to the pressure strip
through JS-6. (A-2) The resulting current flow ranging from 0 to about 1.OmA returns through JS-
7 and is converted to a voltage by US9-1 (C-2). R209 adjusts sensitivity and C1S9 filters out
noise. R147 and D4 prevent overvoltage, (from overpressure) from affecting the
AID
inputs of the
M374S0. D4 (D-2, Schottky diode) clamps any overvoltage to no more than 0.3 volts above the A/D
supply voltage.
2