The second line,
RC,
resets
the 80286 when pulsed low. It
is
used
to
return the processor
to
the real-address mode from protected
mode or
to
reset the system
in
response
to
a Control-Alt-Del.
The
A20_GATE
either enables the 80286 address line A20 (when
high) or forces it
to
a logical 0 (when low). It
is
used
to
cause
80286
addresses
above 1 Megabyte
to
wrap
when operating
in
the real-address mode. Also, the 8041 writes
to
port 69H.
Input Line
The keyboard controller
has
an
input line
(KBINH)
which
is
used
by
the optional keylock keyboard inhibit switch. This front-panel
switch allows keyboard operation
to
be
inhibited by turning the
key
to
the lock position. When locked, the keyboard responses
will still
be
returned, but
scan
codes from the keyboard will
not
be
returned
by
the 8041 .
HP-HIL Interface
The keyboard (and other input devices) are connected
to
the
HP
Vectra
via
a 4-pin
HP-HIL
interface connector.
See
table
4.1
for
the
pin assignments. The
Serial
In
and
Serial
Out
signals are connected
to
the
HP-HIL
controller (U777), which
is
an
integrated circuit
developed by Hewlett-Packard
to
control the
HP-HIL.
The
HP-HIL
controller converts parallel data from the 80286 into serial data
on the
HP-HIL,
and conversely, converts serial data from the
input
devices into parallel data
to
be read
by
the 80286. Approximately
60 times per second, the
HP-HIL
controller polls
the
input
devices
to
read
any input they may ready, and
to
send commands and
data
to
the devices. After receiving data from one or more
input
devices, the
HP-HIL
controller interrupts the 80286
by
asserting
"CERBINT", and then the 80286 reads the data from the
HP-HIL
controller.
Keyboard Interface
57
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