DEC VT220 Technical Manual page 134

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