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5.0.1 W I C A T I C N
MCDE 1
(PC/XT
M e )
DATA
. _ _ _ _ _ _
I
,
START
D O
D1
The
keyboard commicates with t h e h o s t c c p [ p u t e r
using a
synchroncus serial protocol a t approximately 9600BPS.
One
start
b i t , e i g h t data b i t s , no s t o p b i t , and no p a r i t y
are
used t o
make up t h e n i n e b i t data word. When no c c m u n i c a t i o n s
are
i n
progress, t h e keyboard holds the data l i n e low and t h e clock
l i n e high.
/ I
> J
( (
D2
D7
Before s t a r t i n g a transmission, t h e keyboard lowers t h e clock
l i n e as a
R e q u e s t
To Send (RTS). The state of t h e dasa
l i n e is
then checked. If t h e h o s t system is holding t h e &ta l i n e low,
transmissions are disabled. The keyboard w i l l r e t a i n the keyccde
for
t h e pressed key i n its buffer u n t i l t h e clock and data l i n e s
r e t u r n
to
t h e idle state. The keyboard then
r e m s
scanning t h e
keyboard matrix u n t i l t r a n s n i s s i o n s are enabled.
When transmissions are enabled, 100
to
250 microseconds a f t e r
t h e keyboard d r i v e s t h e clock l i n e
low,
t h e keyboard t r a n s m i t s
its data i n t h e previously described
format.
Data is valid
during t h e tire t h e clock l i n e
is
high and f o r a minimum of 10
microseconds a f t e r t h e f a l l i n g edge of t h e clock l i n e . See
Figure
2
f o r a timing diagram.
<
KEYBOARD DATA OUTPUT-XT MODE
>
T1
;
100
-
25011s
T2 ; 104p+20%
T3
;
10ysMIN.
T4 ;
10pMIN.
(CHECKING TIME OF DATA OUTPUT READY OR PROHIBITION)
FIGURE 2
11
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