DEC VT220 Technical Manual page 133

7.4 Detailed Keyboard Circuit Description
This section describes the keyboard circuit. Figure 7-4 shows the
keyboard block diagram.
7.4.1
Keyboard Matrix Scanning
The key locations are arranged in an 18
X
8 line matrix. Each key
switch is connected across a matrix intersection. This gives a
fixed position for each key connected in the matrix. This matrix
accommodates all 105 keys in the keyboard.
Figure 7-5 is a simplified block diagram of the matrix scanning
circuit. Eight lines from PORT 1 of the 8051 microprocessor go to
the binary coded decimal (BCD) inputs of two 74LS145
BCD-to-decimal decoders. Ten outputs from one decoder and eight
outputs from the other decoder provide the drive lines for the
matrix. These 18 lines are called KB DRIVE 0 -- 17.
The other axis of the matrix consists of eight lines tied to +5 V
through pull-up resistors. These lines go to PORT 0 of the 8051
microprocessor and are called KB DATA 0 -- 7.
r - - -
I
PORT
1
I
SCA',S
!
MATR:X
L... _
r - - -
I
PORT 0
I
READS
I
KEY
[,S
8 UNES
2 TYPE
74LS 145
DECODE RS
i8 BCD
INPUTS
AND 18 18 LINES
DECIMAL KB DRIVE 0-17
OUTPUTS}
8 ur;ES
KB DATA
(n
I
1BXBlINE
MATRIX
I
h
I
TYPiCAL. S\\!ITCH
CONNECTION AT
MATRiX
INTERSECTION
Figure 7-5 Matrix Scanning Block Diagram
7-7