ESI VideoBridge 2150, Service Manual

Page: 190 / 332

3 . 4 . 6 V i de o D i splay Ge n e ra tor
T he v i de o d i sp l a y g e ne r a t o r ( V DG ) i s a M o t o r o l a M C 6 8 4 7 w h i c h a c c e p t s a
b i n a ry c o n t r o l word g a te d i n to i t fr om t h e m i c r o c ompu te r .
t o t h a t b i n a ry i n p u t , t h e V D G c r e a t e s c h a r a c t e r s , I n re sponse
g r a p h i c , a n d
s e m i - g r a p h i c i n f o rma t i on t ha t i s outpu t i n a f orma t t h a t c o r r e spond s
t o a norma l t e l e v i s i o n ra s te r s c a n a r rangeme n t .
T h e VDG h a s memory c h i p U 3 1 a s s o c i a te d w i t h i t w h i c h s t o r e s a s c reen
o f i n f o r ma t i o n t o be s ca n n e d and d i sp l a y e d .
T h e v i de o memory h a s tw o
ope ra t i n g mode s . I n i t i a l l y , t h e VDG ha s a c c e s s t o t h e memory a n d i s
a lw a y s s c a n n i ng a nd c o n ve r t i ng t h e memory c on t e n t s t o c ha ra c t e r s t h e n
s e n d s t h o s e c ha ra c te r s o u t t o t he v i de o d i sp l a y . W h e n t he p r oc e s s o r
i s r e a d y t o r e a d o r w r i t e a c h a r a c t e r
t h r oug h a p i e ce o f a r b i t ra t i on l og i c tha t
i n t o t h a t m e m o r y , i t g o e s
sa y s , i f t h e v i de o d i s p l a y
g e n e r a t o r i s d o i n g a h o r i z o n t a l s y n c , a h o r i z o n t a l r e t r a c e , o r a
ve r t i ca l syn c , t h e n g ra n t t h e proce s s or a c c e s s t o t he memory .
Howeve r , i f t he VDG i s n o t do i ng h o r i z o n t a l s y n c o r v e r t i c a l sy n c ,
t h e n i t i s pu t t i ng v i de o o u t a n d mu s t n o t be i n t e r r u p t e d . A t tha t
po i n t t h e p r o ce s s o r i s t o l d to wa i t , t r e a t i n g t h e v i de o memory l i k e
s l ow memory . T h e p r o c e s s o r w i l l w a i t , k e e p i n g the a dd re s s , da ta , a n d
c o n t r o l l i ne s a c t i va t e d , u n t i l a ho r i z on ta l s y n c or a ve r t i ca l s y n c i s
o u t p u t , t h e n t h e m e m o r y r e t u r n s t o n o r ma l o p e ra t i o n a n d r e a d i l y
a c c e p t s new i npu t s f rom t h e p r oc e s sor .
T h e comp o s i t e v i de o , a s i t c ome s ou t o f t he V DG c h i p , i s a v e ry l ow
l e v e l s i g n a l . T h e s i g n a l i s h i g h i mpeda n c e i n na t u re a nd i s ve ry s u s ­
c ep t i b l e to n o i s e a n d i n t e r f e re n ce . To c o u n t e ra c t t h e s e u n d e s i ra b l e
qua l i t i e s , t h e re i s a tw o t ra n s i s tor amp l i f i e r/ bu f f e r
(
Q l , Q2 ,
)
t h a t
t r a n s f orms t h e VDG ou tpu t s i g na l i n to a h i g he r l e ve l , l ow i mpeda n c e
ou tpu t s i g na l . T h i s ou tpu t s i g na l d r i v e s t h e v i de o s e c t i o n o f t h e
2 1 5 0 / 2 1 6 0 .
3 - 2 6
e l s l i 2 1 5 0 / 2 1 6 0 5 / 8 5