Spectravideo SVI 318 Service – Technical Manual page 85

Re a d . A " low'' o n t h i s input p i n enab l e s the
in f o rma t i o n t o t h e
f rom" t he 8 2 55A.
( WR )
W r i t e . A " low" on th i s input p i n enab l e s the
w o r d s i n t o t he 8 2 5 5A .
( Ao a n d A1 )
P o r t S e l ec t 0 and P o r t S e l e c t 1 .
RD
WR
and inpu t s , c on t r o l t he se l e c t i on of o ne o f t he three p o r t s or the
c o n t r o l wor d re g i s te r s .
b i t s o f t he a dd r e s s b u s
8 2 5 5A BAS IC OPERATION
-
A A i RD
0 0 0
0 1
0
1 0 0
0 0
1
0
1 1
1
0 1
1 1
1
X
X
1 0
X 1
( RE SET)
Re s e t . A " h i gh " on t h i s inpu t c l ears the con t ro l re g i s t e r an d a l l por t s
C ) a re s e t t o t he inpu t mod e .
CPU o n t h e d a t a bus .
The s e inp u t s i gna l s , in con j unc t io n w i th the
The y are no rma l l y conne c t e d t o the l ea s t s ig n i f i c a n t
(Ao and A 1 ) .
- -
WR cs INPUT OPERAT I ON ( READ )
0
1 PORT A
1 0 PORT B
0
1 PORT
OUTPUT OPERAT I ON (WR ITE )
0 0
DATA BUS
0
0 DATA BUS
0
0
DATA BUS
0 0
DATA BUS
DISABLE FUNCTION
X
1 DATA BUS
0
1 I LLEGAL CONDITION
1 0
DATA BUS
4 . 52
8 2 5 5A t o send the data or s t a t u s
In e s s e nce , i t a l l ows t h e
CPU t o w r i t e data o r con t ro l
�
BUS
DATA
= = =
DATA BUS
= ===7
;;:;
c DATA BUS
= = =
PORT B
= = =
=?
P ORT c
= = =
�
CONTROL
= =
=9
3-STATE
= =
=9 3-STATE
CPU to " rea d
(A, C ,