Tandy 1000
Technical Reference Manual
8255A18255A·5
8255A OPERATIONAL DESCRIPTION
Mode
Selection
Single
Bit
S9t/Reset Feature
PORT C
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INPUT
o
~
OUTPUT
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PORT C
(UPPER~
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,-
INPUT
_
--- - -
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MODE
SELECTION
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MDDEO
01
=MODE
1
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MODE 2
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Figure
6,
Mode
Definition Format
The
mode
definitions
and
possible
mode
combinations
may
seem
confusing
at
first
but
after
a
cursory
review
of
the
comple_te
device
operation
a
simple,
logical
1/0 ap-
proach
will
surface.
The
design
of
the 8255A has
taken
into
account
things
such
as
efficient
PC
board
layout,
control
signal
definition
vs PC layOut
and
complete
functional
flexibility
to
support
almost
any
peripheral
device
with
no
external
logic.
Such
design
represents
the
maximum
use
of
the
available
pins.
~~--~-----------,
C
MODE 2
-----..:
~~_~'O
l! l
~
! I !I
~~I'DIRECT'ONAL
PB,
PFl"
I/O
~Ol-----.J
PA,PA
o
--Iso
Ill!
C Ill!
{}~
PB]
PB
o
CONTROL CONTROL
PA
7
,PAo
OR
I/O
DR
I/O
~c~_u~=E::==::;-J,
:
~C
.•
;:=~
C:~fr
-~-~---=
I
RD,
WR
07
Do
AoA,
cs
8255A
The modes
for
Port A
and
Port B
can
be
separately defined,
while
Port C
is
divided
into
two
portions
as
required
by
the
Port A and
Port
B definitions.
All
of
the
output
registers, in-
cluding the status flip-flops,
will
be
reset whenever the
mode
is
changed. Modes may
be
combined
so
that their
functional
definition
can
be
"tailored"
to
almost any
I/O
structure.
For
instance;
Group
B
can
be
programmed in
Mode a
to
monitor
simple switch closings or display compu-
tational results, Group A could
be
programmed in Mode 1
to
monitor
a keyboard
or
tape reader on
an
interrupt-driven
basis
When
the
reset
input
goes
"high"
all
ports
will
be
set
to
the
input
mode
(i.e., all 24
lines
will
be
in
the
high
im-
pedance
state).
After
the
reset
is
removed
the
8255A can
remain
in
the
input
mode
with
no
additional
initialization
required.
During
the
execution
of
the
system
program
any
of
the
other
modes
may
be
selected
using
a
single
output
instruction.
This
allows
a
single
8255A
to
service
a
variety
of
peripheral
devices
with
a
simple
software
maintenance
routine.
There
are
three basic modes
of
operation that
can
be
select-
ed
by the system software:
Mode a - Basic
Input/Output
Mode 1 - Strobed
Input/Output
Mode 2
--
Bi-Directional Bus
Figure
5,
Basic
Mode
Definitions
and Bus
Interface
Any
of
the eight bits
of
Port C
can
be Set
or
Reset using a
single
OUTput
instruction.
This feature reduces software
requirements in Control-based applications.
6-169
AFN.Q0744C
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