Table of Contents
Advertisement
Strobe Synchronization
The horizontal sync signal triggers the strobe which
sequentially illuminates the IR LEOs. If positive sync
is configured, the rising edge of the signal triggers the
strobe and if negative sync is configured, the falling
edge triggers the strobe. There are 32 IR LEOs
mounted across the vertical plane (left side), and 32
across the horizontal plane (bottom) of the front bezel
assembly. Each of 64 distinct bytes of data decode
and drive each one of the IR LEOs.
With respect to the front bezel, the strobes occur in
a counter clockwise direction beginning with the verti-
cal plane, followed by the horizontal plane. The hori-
zontal synchronization signals are supplied to the logic
board from pin 1 (negative sync) and pin 2 (positive
sync) of monitor input connector J7.
If positive horizontal synchronization is configured, the
signal goes to pin 12 of IC106. Diodes CR112 and
CR113 limit the signal to -0.7 to +5.7 volts. Pin 11
of IC106 is held high by pull-up resistor R120. On each
high-to-Iow transition of the positive sync pulse, IC106
sends a 10 j.LS pulse to pin 39 of microprocessor
IC101.
If negative horizontal synchronization is configured,
the signal goes to pin 11 of IC106. Pin 12 is held low
by R121 which is connected to ground. On each low-
to-high transition of the negative sync pulse, IC106
sends a 10 j.Ls pulse to pin 39 of microprocessor
IC101.
These pulses synchronize the IR LED strobe se-
quence.
Infrared LED Strobe Sequence
To implement the strobe, sequential data bytes decode
which of the IR LEOs to illuminate in the vertical plane,
and then the LEOs in the horizontal plane. Each byte
Page
3-6
of data is placed onto the OB7 - OBO bus and is also
temporarily stored in a register within the microproces-
sor. When data on the bus is valid, port P16 goes
low providing the chip enable signal to pin 1 of IC108.
The low at port 16 also forces the output of
NAND
gate
IC105 high. Pin 2 of inverter IC104 inverts the Signal
and enables IC106. Microprocessor IC101 then out-
puts an active write (WR*) pulse. The low-to-high
transition on the lagging edge of the pulse causes octal
flip-flop IC108 to latch the status of data on the DB7
- DBO bus to its Q outputs.
The active write cycle also produces a high at pin 6
of IC1 06. This high enables each of the inputs of
NAND
gates IC109 and IC110 to conduct the latched output
of IC108. This output enables one set of 8 IR LEOs.
By holding the cathodes of each set of IR LEOs low,
each anode can then be selectively strobed, causing
each of the 8 IR LEOs to be sequentially illuminated.
Data on the DB7 - DBO bus is used to strobe the
anodes of the enabled group of IR LEOs through buf-
fer/driver IC107.
After an IR LED is decoded and strobed within the
vertical plane, IC101 checks for the presence of a
finger on the CRT by reading the status of its testable
input at pin 1. If a finger intersects the infrared projec-
tion a low appears at the testable input. The micropro-
cessor then retains the byte stored in its internal regis-
ter corresponding to that particular location as the X
coordinate data.
The microprocessor then decodes and strobes each
IR LED within the horizontal plane. When the testable
input is driven low by the placement of a finger on
the CRT, the microprocessor retains the data byte
stored in its internal register corresponding to that par-
ticular location as the Y coordinate data.
With X-Y coordinates established, the microprocessor
interprets where the finger has been placed on the
screen. The microprocessor uses the X-Y coordinate
data bytes to execute an instruction contained within
Circuit Descriptions
Advertisement
Table of Contents
