Tandy 2000 Service Manual page 104

Tandy@
Model
2000
Service
lflanual
"1", the logic level is translated to about
+12 Vdc
by a
751488
level shifter.
this in turn drives the gate of an
IRFD110
HEXFET,
allowing current to pass from drain to source and on to
the
keyboard Vcc
input.
Driving
-I2
Vdc
into
the gate
of
the
HEXFET
(logic rr0'r
translated by the
751488)
turns the transistor
and the keyboard off.
7
.L.10
,
3
Revision Port
To
read
the revision
port,
Port
C
bits I
and
2
musL
be
programmed
to a "10".
This enables the
74L5244
buffer for output onto the
Port B data bus.
The
revision port is
encoded
with an 8-bit
number
reflecting
the current revision level of the main logic
board.
Each
PCB
update
will
increment
this 8-bit
value by
one.
95