Tandy 2000 Service Manual page 613

8255A/82554-5
Input Control Signal Definition
STB
(Slrobe Input). A
"low"
on
this input loads data into
the input
latch.
IBF
(lnput Buffer Full
FrF)
A "high"
on
this output
indicates
that
the data
has been
loaded
into
the
input latch;in
essence,
an
acknowledgement
IBF
is set
by
STB
input
being
low
and
is reset
by
the
rising
edge
of
the RD input.
INTR
(f
nterrupt
Request)
A "high"
on this
output
can be used
to interrupt
the
CPU
when
an
input device is requesting seruice.
INTR
is set by
the SfB
is a
"one",IBF
is
a "one"
and INTE
is
a
"one".
It
is reset
by the falling
edge
of
RD. This procedure allows
an
input
device
to
request
service
from the
CPU
by
simply
strobing its data into the
port.
INTE
A
Controlled by
bit
set/reset
of
PC+.
INTE
B
Controlled by bit
set/reset
of
PC2.
INPUT
FROM
PERIPHERAL
CONTROL WORD
D7 D6 D5 D4
D3
CONTROL WORD
D7
D6 D5
D4 Dr
DZ
Do
D2
PCe. t
I
= INPUT
0 = OUTPUT
sTBA
tBFA
INTRA
r/o
srBE
tB
FB
INTRts
Figure
8.
MODE 1
Input
Figurc
9.
UODE
1
(Strobed Input)
PA7-PAo
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lal
J--',
ir
+__ t51
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V
*
t51s
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I
tsrrf'F
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f
<
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6-174
AFN-00744C