Tandy
1000
Technical Reference Manual
8255A/8255A·5
PROGRAMMABLE
PERIPHERAL
INTERFACE
• MCS-85™ Compatible 8255A·5
•
24
Programmable I/O Pins
• Completely TTL Compatible
• Fully Compatible with
Intel~
Micro-
processor Families
• Improved Timing Characteristics
• Direct Bit Set/Reset Capability Easing
Control Application Interface
• Reduces System Package Count
• Improved
DC
Driving Capability
• Available
in
EXPRESS
-Standard
Temperature Range
-Extended
Temperature Range
The
Intel~
8255A
is
a general
purpose
programmable
I/O device
designed
for use
with
Intel'
microprocessors
It has
24 I/O
pins
which
may be
individually
programmed
in 2
groups
of
12
and used in 3
major
modes
of
operation.
In
the
first
mode (MODE
0),
each
group
of
12
I/O
pins
may be
programmed
in
sets
of
4
to
be
input
or
output.
In MODE
1,
the
second
mode, each
group
may be
programmed
to
have 8
lines
of
input
or
output.
Of
the
remaining
4
pins,
3 are
used
for
hand·
shaking
and
interrupt
control
signalS The
third
mode
of
operation
(MODE
2)
is a
bidirectional
bus
mode
which
uses
8
lines
for a
bidirectional
bus, and 5
lines,
borrOWing one
from
the
other
group.
for
handshaking
o
>r~ll
t;r
~
-8?;:
-{~}
L
---.
-------
j
u
Figure
1.
8255A Block Diagram
6-166
Figure
2.
Pin Configuration
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