Tandy 1000 SL Technical Reference Manual page 252

Before s t a r t i n g a transmission, t h e keyboard checks t h e s t a t u s
of t h e clock and data l i n e s . Transmissions are disabled i f t h e
clock l i n e
is l o w
and t h e code f o r t h e pressed key is held
i n
t h e keyboard buffer u n t i l transmissions
are enabled.
I f t h e
clock l i n e is high and the
data l i n e is low, t h e h o s t system is
sending a R e q u e s t
To
Send (RTS), and t h e code f o r t h e pressed
key is stored i n t h e buffer.
When transmissions are enabled and no RTS is detected f r m t h e
host system, both clock and data l i n e w i l l
be
high.
T k
keyboard
starts a transmission by sending a low start b i t , followed by
t h e rest of t h e data word. Data is v a l i d 20 microseconds minimum
prior
t o
t h e f a l l i n g edge of t h e clock. See Figure 3 f o r timing
diagram.
During t h e transmission of a data word, t h e keyboard
p e r i o d i c a l l y checks t h e
state of t h e clock l i n e . I f t h e clock
l i n e is low during t h e s e checks p r i o r to
t h e r i s i n g edge of t h e
p a r i t y b i t , a d a t a c o l l i s i o n
occurs.
When a data c o l l i s i o n
occurs,
t h e keyboard s t o p s transmitting, r e t u r n s t h e code f o r
t h e pressed key t o t h e keyboard buffer, and prepares to respond
t o a c t i o n s requested by t h e host system.
5.1
COMMANDS
FROM THE KEYBOARD TO THE H O S T
S Y S T E M
Keyboard Buffer Overrun
-
AT
and
XT
&des
This buffer can store up t o
s i x t e e n codes f o r pressed keys. When
t h i s buffer is f u l l
and
t h e seventeenth code is received, t h i s
code is replaced by an Overrun Code. The following c h a r t shows
t h e Overrun Codes f o r each d e .
Codes received after t h i s code are lost u n t i l t h e keyboard
clears additional space i n t h e keyboard buffer.
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