Tandy 2000 Service Manual page 608

!ru
8255A/82554-5
8255A OPERATIONAL DESCRIPTION
Mode
Selection
There are three
basic modes
of
operation
that
can be
select-
ed by the system
software:
Mocle
0
-
Basic
Input/Output
Mode
1
-
Strobed
InputlOutput
Mode2 -
Bi-Directional
Bus
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)"
Af
ter the reset is removed the 8255A can
rernain 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
rout ine.
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
fl ip-f
lops,
will
be
reset whenever the
mode
is
changed. Modes
may be combined so
that
their
functional definition
can be
"tailored" to
almost any l/O
structure.
For
instance;
Group
B
can be programmed
in
Mode 0
to monitor
simple switch closings or display compu-
tational
results,
Group
A
coulcl be programmed
in
Mode
1
to
monitor
a
keyboard or tape
reader
on
an
interrupt-driven
basis.
c
MoDrt
-
4
J
Jalto I I I |
| | | |
lsltro
-.?
trl+ rri*
<-j'
PB7
PBo
tBHt'?8t tBtt'?o"t
PA7
PAo
r\4ODE
r
=#
1=,
P ro
r I t i | | r I
lslBI'DIRECTIONAL
rrt{
I 1l
<J
PB7
PB"
t/O -;;_fi*-
PAT"PA'
Figure
5,
Basic Mode Definitions
and Bus
lnterface
CONTROL
WORD
MODE
SET FLAG
1 =
ACTIVE
Figure
6.
Mode De{inition Format
The mode
def
initions and
possible
mode combinattons
may seem conf using at
f
irst but
af
ter
a
cursory review
of
the complete device operation a
simple, logical
l/O
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.
Single Bit Sgt/Reset Feature
Any
of
the eight bits
of
Port
C can be Set
or
Reset using
a
single
OUTput instruction. This
feature reduces software
regu
irements in Control-based applications.
GROUP
B
PORT C (LOWENI
1 = INPUT
0 = OUTPUT
PORT B
1 = INPUT
0 = OUTPUT
MODE SELECTION
0=MODE0
1=MODE1
GROUP A
PORT C (UPPER)
1 = INPUT
0
=.
OUTPUT
PORT A
I
= INPUT
0 = OUTPUT
MODE
St.
LECTIO\I
00
-
MoDE
0
01
-
MODE
'l
1X = MODE
2
6-169 AFN-00744C