Principles of Operation
6-12
6.6
Light pen interface
The light pen interface circuitry is on the front panel control
board. A simplified schematic is shown in Figure 6-10.
The light pen is a self-contained unit requiring a 5 volt sup-
ply. It returns the following two TTL-compatible signals:
• SENSEPULSE — A low-going pulse of about 15µs
width that occurs as the light intensity at the end of the
pen rises above a preset threshold.
• LPSWITCH — A debounced switch signal that is low
while the light pen pushbutton is depressed.
The rising edge of SENSEPULSE clocks a high into flip-flop
U130 (74HCT74). This converts the signal into a level that
can be read by the microprocessor through the PB7 input of
U103 (6522A VIA) as signal LPSENSE (low true).
If the low true SLAVE signal from U108 (63821
PIA)($3850-385F) is high (i.e., the unit is either a stand-
alone or master), the microprocessor reads its own U130 flip-
flop. The output of U130 also gets driven onto the LPSENSE
line of the master/slave connectors. If SLAVE is asserted, the
microprocessor reads the LPSENSE signal from the master/
slave connectors.
The microprocessor can clear flip-flop U130 by setting the
LPRESET output of U103 high. This signal also gets driven
onto the master/slave connectors by open-collector driver
U134. This method permits the master and all slaves in a
master/slave system to read and clear the U130 flip-flop in
the master unit. Thus, one light pen can serve for all units,
while each unit controls its own display for the scan routine.
The switch signal (low true LPSWITCH) goes to the CA1
interrupt input of the VIA, which is programmed to generate
an IRQ interrupt on the falling edge of LPSWITCH. The
interrupt service routine stops the normal display refresh
multiplexing and takes over control of the display.
The routine then scans the display one column at a time,
clearing flip-flop U130 before scanning each column. After
the display is scanned, the processor examines the
LPSENSE signal to determine if the light pen “sees” one of
the LEDs that is presently being scanned.
If a master scans its display and receives no response from
the light pen, it instructs the slaves to scan their displays.
Each slave monitors the U130 flip-flop of the master to check
whether or not the light pen “sees” any of the LEDs that are
lit on its own display.
Figure 6-10
Light pen interface simplified schematic
HCT74
U130
D
Q
CLR
U136
HCT244
HCT125
U126
+5V
+5V
HCT125
U126
U134
LPSENSE
LPRESET
LPSWITCH
SENSEPULSE
MASTER/SLAVE
CONNECTORS
LIGHT
PEN
CONNECTOR
63821
PIA
PA2
SLAVE
U134
HCT244
U136
U108
LPSENSE
LPRESET
LPSWITCH
6522A
VIA
U103
PB7
CA2
CA1
IRQ
TO
68B09
IRQ
+5V
'38
'38
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