Spectravideo SVI 318 Service – Technical Manual page 82

B u s D I Rec t i on , B u s Co n t ro l
The s e b us c o n t ro·l s i g n3 l s a r e g e n e r a t e d d i r e c t l y by Z80A s e r i e s o f m i c r o ­
p r o c e s s o r s t o c o n t r o l
u s i n g a p ro c e s s o r
Whe n
e i t h e r b y compa r a b l e b u s s i g n a l s o r b y
t h e p o r c e s s o r .
Th i s c ou l d s i mp l i fy w ou l d
f o l l ow i n g , wh i c h
c o n t r o l s i g n a l s ( B D I R
B D I R BC2 B C l
0 1 0
0 1 1
1 0
1 1
ANA LOG C HANN EL A , B , C ( o u t p u t )
E ac h o f t h e s e s i g na l s i s th e ou t p u t o f i t s c o r r e s p o n d i n g m i x o f T4 , a n d
p ro v i d e s a n up t o 1 V p e ak-p eak s i g n a l rep r e s e n t ing t he comp l e x s ound wave shape
g e n e ra t e d by t h e PSG .
I OA 7 -- I OAO ( i n pu t / ou t p u t )
I OB 7 -- I OBO ( i n pu t / ou t pu t )
Eac h o f t h e s e two p a r a l l e l i n p u t / ou t p u t po r t s p ro v i d e s 8 b i t s o f par a l l e l data
t o / f rom t he P S G / C PU b u s f rom / t o any ex t e rna l d e v i c e s conne c t e d t o the l OA or
lOB p i n s . Eac h pin i s p ro v i d e d w i t h an on-c h i p pu l l - up re s i s t o r , s o tha t when
i n t he " i npu t " mod e , a l l p i ns w i l l rea d norma l l y h i gh .
re c omme n d e d me t hod for sc ann i n g e x t e rna l swi t c he s , for examp l e , wou l d be t o
g r o und t he i n p u t b i t .
TES T
p i n 3 9
T EST 2 p in 26
T h e s e p i ns a re f o r us e r t e s t pu rpo s e s o n l y and s h o u l d be l e f t open - do not use
a s t i e - po i n t s .
p in 40
N om i n a l Vo l t power s u pp l y t o the PSG .
+ 5
Vs s
p i n 1
G round re ference for the PSG .
1
2 ,
e x t e rna l and
a l l
than Z 8 0A ,
o t h e r t h e
p ro g r a mm i n g o f t he b u s c o n t r o l s ig na l s t o t h e
t he
o n l y re q u i re
t h a t
BC l , w i th BC2 t i e d to + 5V ) :
a n d
FUN CT I ON
P S G
INACT IVE .
READ FR I N P S G .
WRITE TO PSG .
LATCH ADD R E S S
p i n s 1 4--2 1
p i n s 6 -- 1 3
4 . 49
b u s op e ra t i o n s in the P S G .
i n t e r n a l
s i m u l a t i n g s i gna l s c a n b e p rov i d e d
t h e s e
t h e s i g na l s on I / 0 l i n e s o f
p r oc e s s or g e ne r a t e t wo b u s
t he
FROM
- P RO CE S SOR
T h e re f o re , the
PSG
B D I R
B C 2
B C l