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p u r p o s e o f t h e D C U n s a f e c i r c u i t i s to m o n i t o r t h e -f 2 4 - v o l t a n d + 5 - v o l t l e v e l s a n d
c o m p a r e e a c h l e v e l w i t h a p r e c i s e r e f e r e n c e v o l t a g e .
e x c e e d e d , + 2 4 v o l t s is t u r n e d o f f to d i s a b l e t h e w r i t e a n d e r a s e l o g i c ( s e e
F i g u r e 3 - 2 6 ) .
W r i t e C u r r e n t C o n t r o l
T h e w r i t e c u r r e n t c o n t r o l l o g i c is s h o w n in F i g u r e 3 - 2 7 .
c o n t r o l t h e f l o w o f w r i t e c u r r e n t t h r o u g h t h e r e a d / w r i t e h e a d in r e s p o n s e to t h e
d i r e c t i o n d e t e r m i n e d b y t h e W R T D A T A i n t e r f a c e l i n e .
W h e n t h e d r i v e is n o t s e l e c t e d , w r i t e c u r r e n t f l o w is i n h i b i t e d .
s e l e c t e d , i n t e r f a c e l i n e r e c e i v e r 5A is e n a b l e d a n d g a t e s W R T D A T A to w r i t e f l i p -
f l o p 3D.
I f t h e W R I T E c o m m a n d is a c t i v e , t h e f l i p - f l o p is e n a b l e d a n d t h e o u t p u t
o f Q 6 d r i v e r 2D i s lo w.
c u r r e n t s o u r c e t r a n s i s t o r Q 6 to t u r n o n.
s o u r c e t r a n s is to r ' Q 5 ' is' t u r n e d off.
W h e n W R I T E b e c o m e s i n a c t i v e a n d t h e f l i p - f l o p t o g g l e s to t h e o p p o s i t e s t a t e , Q5
d r i v e r 2D o u t p u t s l o w , a n d c u r r e n t f l o w s t h r o u g h V R 2 , C R 3 , R 6 l a n d R 6 2 c a u s i n g
C u r r e n t f l o w s t h r o u g h V R 2 , C R 3 , R b 3 , a n d R b 4 c a u s i n g
Q5 d r i v e r 2D o u t p u t s a h i g h a n d c u r r e n t
F i g u r e 3 - 2 6 .
DC U n s a f e L o g i c
3 - 2 9
If t h e v o l t a g e p a r a m e t e r s a r e
T h i s c i r c u i t is u s e d to
W h e n t h e d r i v e is
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