Spectravideo MKI, Service – Technical Manual

Page: 82 / 134

Bus DI Rec t i on , Bus Co n t ro l
2 ,
1
The s e bus cont ro·l s i gn3 l s a r e genera t e d d i re c t l y by Z80A s e r i e s o f m i c ro­
p roce s s o r s t o cont r o l
a l l
e x t e rna l and
i n t e r n a l
bus ope ra t i ons in the P S G .
Whe n
us i ng a p ro c e s s o r
o t h e r
than
t h e
Z 8 0A ,
th e s e
s i gna l s c a n b e p rov i d e d
e i t h e r by compa rab l e b u s s i g na l s or by
s i mu l a t i n g
t h e s i gna l s on I / 0 l in e s o f
t he porce s s o r .
Th i s c ou ld simp l i fy
t he
p ro g ramm ing of the b u s c o n t r o l s ig na l s t o the
f o l l ow ing , wh ich
w ou l d
on l y re qu i re
t h a t t he
p roce s s or g e ne ra te two bus
c on t r o l s i gna l s ( B DIR
a n d
BC l , w i th BC2 t ie d to
+ 5V ) :
BDI R
BC2
BCl
PSG
FUNCT I ON
PSG
0 1 0 INACT IVE . FROM B D I R
0 1 1 READ FR I N PSG .
- P RO CE S SOR
BC2
1
0 WRITE TO PSG .
BCl
1 1 LATCH ADDRESS
ANA LOG CHANNEL A, B , C ( ou t p u t )
Eac h o f these s i gna l s i s th e ou t p u t o f i t s c o r r e sponding m i x o f
T4 ,
and
p ro v i d e s an up to 1V p eak-peak s ignal rep r e s e n t ing t he comp l ex s ound wave shape
gene ra t e d by the PSG .
I OA 7 -- IOAO ( i npu t / ou t p u t )
I OB 7 -- I OBO ( i npu t / ou t pu t )
A7 --AO , B 7 --BO
p i n s 1 4--2 1
p i n s 6-- 1 3
Eac h o f t h e s e two p a r a l l e l inpu t / ou t p u t po r t s p ro v i d e s 8 b i t s of para l l e l data
to / f rom t he PSG/ CPU bus f rom/ to any ex t e rna l d e v i c es conne c t e d to the lOA or
lOB p i n s . Eac h pin i s p ro v i d e d w i th an on-ch ip pu l l -up re s i s t o r , so 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 ly h i gh . The re f o re , the
re comme nded me t hod for sc ann ing e x t e rna l swi t c he s , for examp l e , wou l d be to
g ro und t he input b i t .
TES T p i n 3 9
T EST 2 p in 26
The s e p i ns a re f or us er te s t pu rpo s e s only and should be l e f t open - do not use
as t i e-po i nt s .
p in
40
Nom in a l
+ 5
Vo l t power supp l y to the PSG .
Vs s
p in 1
G round re ference for the PSG .
4 . 49