DEC VT220 Technical Manual page 134

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The
8051
scans
the 18 drive lines. Key closures are detected by
reading the eight data lines. The complete matrix is scanned every
8.33 ms.
When a key closure is detected, it is scanned again to verify that
it is really a key closure and not just electrical noise.
Once the key closure is verifi
,
position
information into a key
Transmission
is
handled
by
the
Transmitter (UART) in
8@51.
8@51 firmware translates the
e and transmits it to the CPU.
Universal Asynchronous Receiver
A ghost key indication can occur
ee of the four corners of
a matrix
rectangle are closed (Figure 7-6), causing what is known
as
a
sneak path. The key positions in the matrix are arranged to
avoid
sneak
paths.
However,
if
a
sneak
path does occur, the
firmware
prevents
keycode fo
key that caused the sneak
path to be transmitted until one of the involved keys is released.
This prevents transmission of
t
entirely.
Table
7-1
shows
the
keyboard matrix on the LK201-AA (American)
keyboard.
Keycap
designations are lis
for reference only; you
can compare them to Figure 7-7 (A and B).
DRiVE
LINES
DATA
LINES
1 CONDITIONS ARE
SWITCHES 82, 83, AND C3 CLOSED,
SWITCH C2 OPEN, L,INE 215 BEING DRIVEN AND LINE C
IS BEING READ
INTERSECTION C2
BEING LOOKED AT. IT SHOULD NOT
KEY CLOSURE BECAUSE SWITCH C2 IS OPEN
3 HOWEVER A SNEAK PATH is PRESENT FROM LINE 2
THROUGH SWiTCHES 82, 83, AND C3 TO liNE
A GHOST KEY is READ AT INTERSECTiON C2,
Figure 7-6
Example of Ghost Key Generation
7-8

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