Tandy 2000 Service Manual page 619

intel'
8255A/9255A,5
APPLICATIONS OF THE
8255A
The 8255A
is a
very
powertu
I tool
f
or
interf acing
peripheral equipment to the
microcomputer system.
lt
represents the optimum
use
of available pins and
is
f
lex-
ible
enough
to
interface almost any llO
device
without
the
need for additional external logic.
Each peripheral device
in a
microcomputer
system
usually
has
a "service routine" associated
with it,
The
routine
manages
the software interface
between
the
device
and the
CPU.
The functional
def
inition of
the
8255A
is
programmed by the llO
service
routine
and
becomes
an extension
of the
system software.
By
ex-
amining
the
l/O devices interface
characteristics for
both data transfer
and
timing,
and
matching this
inf or-
mation to the examples
and
tables
in
the detailed
opera-
tional
description,
a control
word can easily be
devel-
oped
to
initialize the
8255A
to
exactfy
"fit"
the
applica-
tion.
Figures 19
through 25 present a few
examples of
typical applications of the
8255A.
INTERRUPT
REOUEST
INTERRUPT
REOUEST
INTERRUPT
REOUEST
Figure
21.
Keyboard and Terminal Address
Interface
T
PC:
PA^
v
B2ssA
PAr
PAz
PAr
PAq
PA"
il3P.i'l
.
'o;
PAz
PCq
jtu
' ,"0
PBr
PBz
PB:
PBa
PBE
'#rT'i}'
PBu
PBz
PCr
PCz
PCe
lct
PCo
+-
<-
Ro
Rl
R2
FULLY
R^
DECODED
,
KEYEOARD
R4
R-
SHIFT
CONTROL
STROBE
ACK
Bo
B1
FI BURROUGHS
SELF
SCAN
83
DISPLAY
B4
B5
BACKSPACE
CLEAR
DATA
READY
ACK
BLANKING
CANCEL
WORD
PT
PC:
PA^
PAr
PAz
PA:
PAc
PA.
,8'lliJ,+'
I
'o.
PAr
PCr
PCe
MODE'I
(OUTPUTI
DATA
READY
ACK
PAPER FEED
FORWARDIREV
DATA
READY
ACK
HIGH
SPEED
PR
INTE
R
HAMMER
RELAYS
DATA BEADY
ACK
PAPER FEED
FORWARD'REV
RIBBON
CARRIAGE SEII
INTER
R
UP
REOUEST
Figure 20,
Keyboard and Display Interface
INTERRUPT
REOUEST
TE
RMINAL
ADDR
ESS
MODE
1
IINPUTI
E255A
PAo
PAr
PAz
PA:
PAa
P\
PAo
PAz
PCo
PCs
PCe
PCz
MODE
O
(INPUT}
Ro
R1
FI
^
FULLY
^3
DECODED
R4 KEYBOARD
R-
f
SHIFT
CONTROL
STROBE
ACKNOWLEDGE
BUSY LT
TEST LT
Figure
19. Printer
Interface
6-180
AFN-007.'+4C