Spectravideo MKI, Сервисное – Техническое руководство, Страница: 84 / 134

THE PROGRAMMABLE PERIPHERAL INTERFACE.
8255A
The 8 2 5 5A PP I i s u s e d to s t robe the ke yboa r d l i ne s , to int e r face the j oy s t ick
f i re b u t t on swi t c h , the p a dd l e and to cont ro l the ca s s e t t e tap e system. The
por t s a re p rogramme d as f o l low :
Po r t A
Po r t B
P o r t C
Prog ramme d as input por t . B i t
0-3
are u s e d for the pa dd l e i npu t ,
b i t s 4 and 5 a r e u s e d for the j o y s t i ck f i re sw i t ch i np u t and b i t s
6 and
7
a re u s e d f o r s igna l s f rom t he c a s se t t e .
P rogrammed as ou t p u t port and i s u s e d for keyboa rd read data .
P rog ramme d a s output p o rt . B i t s
0-3
ou tpu t s BCD d a ta f or keyboard
scann i ng . B i t s 4-6 ou t p u t s con t ro l s i gna l s for the ca s se t t e . B i t
7
i s used to m i x PSG s ound da ta .
The func t i ona l co nf i g u ra t ion of the 8 2 55A i s p ro g ramme d by the system sof twa re
so t h a t norma l ly no ex t e rnal log i c i s nec e s sary to i n t e r f ace p e r iphe ra l d e v i c e s
or s t r uc t u r e s
•
Da t a Bus Buf f e r
Th i s 3 -s t a t e b i d i re c t i ona l 8-b i t buf f e r i s us e d to in t e r f ace the 8 2 55A to the
s y s t em da ta b us . Data i s t ransm i t te d or rece i ve d by t he b u f f e r upon execu t ion
of input or output i n s t r uc t i ons by the CPU . Con t ro l wo rds and s ta t us infor­
ma t i on a re a l so t rans f e r re d t h rough the d a ta bus bu f f e r .
R ea d /Wr i t e and Cont r o l
The func t ion· of th i s b lock is to ma nage a l l of the int e rnal and e x t erna l
t ra n s f e rs of both Da ta and Con t r o l or S t at us word s . I t accep t s i npu t s f rom the
CPU Add re s s and Cont ro l bus s e s in turn , i s s u es commands to bo th of the Con tro l
G r oup s .
( C S )
Chip S e l ec t . A
"
low
"
on th i s input pin enab l e s the commun i c a t i on be tween the
8 2 55A and the CPU .
4 . 51