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Theory of Operation
1780R-Series Service Manual
3–63
MPU enables U437, Read (RD) enables the buffers to read from the bus, and
Write (WR) enables the buffers to write to the bus. RL0-RL7 are data inputs
from the Front Switch Panel (Diagram 23) and Touch Panel (Diagram 24).
SL0-SL3, OA0-OA3, and OB0-OB3 are outputs to the Front Switch Panel
(Diagram 23) and Touch Panel (Diagram 24).
Drivers. U338 is a 3-line to 8-line decoder used to decode SL0-SL2 in order to
provide 1 of 8 mutually exclusive column select signals for the front-panel
drivers. Row information to complete the switching matrix of the Keyboard/Dis-
play is returned to R0-R4. CR106 through CR113 in the SCANL lines from
U338 prevent false imaging when multiple switches are pushed.
U140, U239, and U240 are high power cathode drivers for the front-panel
indicators (LEDs) and the touch panel LEDs. Anode voltage for the LEDs is
provided by U223. U210 regulates the 12 volt supply to account for the changing
current requirements of the front-panel LEDs; without an external regulator the
LEDs would dim and brighten according to the number lit. Up to eight LEDs can
be lit by one select line. R208 through R215 provide current limiting. Resistor
values for the front panel and the touch panel are different, because the touch
panel LEDs are lit in pairs, not singly.
Control PAL. U508 outputs the TP0 and TP1 (TANOD1 and 2 enables for U223)
and a LATCH signal to clock the Serial/Parallel register. The cycle starts with
BDL from U437 going high. BDL is inverted (U520C) and input to U508 and at
the same time resets counter U512. U512 counts up to 256. When both count 64
and count 256 are high TP0 goes high and U223 outputs TANOD1 to the
Waveform Monitor touch panel. The delay of 256 clock cycles is required to
ensure that each LED is read at 50% duty cycle which increases the touch panel
noise margin. BDL cycles a total of eight times in order to read all eight LEDs
on the CRT touch panel. Each time a touch panel LED is read its state is output
(TP OUT), inverted (U520A), and input to serial input of the Shift Register
(U420).
After eight cycles the LATCH output of U508 goes low in preparation to scan
the Vectorscope touch panel. When LATCH goes high the cycle starts over again
with TP1 alternately going high and low for eight cycles to read the Vectorscope
touch panel LEDs and input the data into its Shift Register (U413). After two
complete cycles the LATCH data from both touch screens is contained in the
serial register of the Shift Registers.
On the second LATCH transition, data from the serial registers is transferred into
the parallel registers of U413 and U420. The Master MPU reads the touch panel
information from U413 when a read instruction is sent to the CSTPL address and
A0 is low. U420 is read when A0L (from U508) goes low along with the read to
CSTPL.