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T a b l e 5 . 1
DRC S i g n a l s and F u n c t i o n s
S i g n a l
F u n c t i o n
a ) P i n 1 2 of U 2 1
Clock s i g n a l of A/D. c o n v e r t e r .
Frequency
I f n o t p r e s e n t
should b e a b o u t 150 K Hz.
r e p l a c e U 7 .
Also check R22 and C36A.
I n t e g r a t e d s i g n a l of A/D t o d e t e r m i n e t h e
count p e r i o d .
The p e r i o d s h o u l d b e a b o u t
.8 s e c o n d s .
I f n o t p r e s e n t , check* U21
and C30.
b ) P i n 1 4 of U20
c ) S i g n a l a t R4 ( c l o s e s t t o
T h i s i s t h e m i c r o p r o c e s s o r s i g n a l between
U16)
t h e CPU (3850) and SMI (3853).
The s i g n a l
should p u l s e once e v e r y second.
I f t h i s s i g n a l i s n o t p r e s e n t , check T a b l e
5 . 1 ,
s i g n a l d ) , e ) , f ) .
I f t h i s s i g n a l
i s
p r e s e n t b u t does n o t p u l s e , U 1 8 and U19
should b e checked f o r p r o p e r o p e r a t i o n .
U18 i s a quad two-input NAND g a t e (7400)
and U19 i s a hex i n v e r t e r (7404).
Checking l o g i c l e v e l s of t h e s e I C ' s w i l l
i n s u r e p r o p e r g a t e o p e r a t i o n .
I f b o t h
U 1 8 and U19 a r e working, see T a b l e 5 . 1
s i g n a l s d )
,
e ) , and f )
.
d ) P i n 40 of U 1 7
Microprocessor RC c l o c k s i g n a l .
I f n o t
p r e s e n t , check v a l u e s and c o n n e c t o r s f o r
R2, R 3 , and C37.
The f r e q u e n c y should b e
a b o u t 250 K Hz.
I f t h e c a p a c i t o r s and
r e s i s t o r s are good, check t o b e s u r e t h a t
t h e +12 v o l t s u p p l y t o t h e RC network
i s
p r e s e n t
.
e ) P i n 1 of U 1 7
P i n 2 of U16
Clock l i n e from CPU (3850) t o SMI (3853).
I f n o t p r e s e n t , m i c r o p r o c e s s o r i s n o t
c l o c k i n g t h e i n t e r f a c e d e v i c e .
Replace U17.
f ) P i n 2 of U 1 7
W r i t e l i n e from CPU t o SMI.
I f n o t p r e s e n t ,
P i n 3 of U 1 6
r e p l a c e U 1 7 .
*
Note t h a t t h e e a s i e s t way t o check on
I.C.
i s by d i r e c t r e p l a c e m e n t .
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