DEC VT220 - 7.3 Functional Description

7.3
FUNCTIONAL DESCRIPTION
This section provides a
keyboard.
functional description of the LK20l
7.3.1
Keyboard Operation Overview
Figure 7-4 is a simplified block diagram of the keyboard
circuitry. Everything except the block marked KEYBOARD SWITCH
MATRIX is on the printed circuit board. This block represents the
connections between the keyboard switches and the signals from the
8051 microprocessor.
+5V
NFORMS
RS-423
_·10V
-IOV
SUPPLY
+5V LEO
SUPPLY
-
BEEPER
BEEPER ]" JL BEEP=KEYCLICK
CONTROL 2 MS
CIRCUiT Y ~BEEP.BELL
1 2KHz 125MS
KEYBOARD
LEOs
CONTROL
CIRCUIT
..------.-,.-~=---,J
TIMER
TIMING NO 1
NETWORKS ~-T-IM-E-R--" 190 KHz
- NO.2
,
I
JUMPERS FOR
I
HARDWARE 10
I
i
....
r"'
RECEIVE
RECEIVER
"""" }
CO
TRANSMIT
TO
DRIVER
SERIAL OUT
1
t
-IOV
+12V
+5V
I--
SUPPLY
GND
'-'
-1
r-
P I
I
P
0,
1
0
4
~ I
I R
1
I
I T
_J
I
2
I
I
I
3
I
~-
ON/OFF
07
8
CONN
,,-., KB
DAiA
8
4KB ROM
128 BYTES RAM
pi
a'
" I
T i
o I I.
_J 8051 i
• MICRDPROCESSO~ I
r -I
I
I P
1
0
I R
IT
I 3
I
I
I
~---.{RfSET I
I
L_
18
BCD TO
DECIMAL
DECODER
POWER UP
RESET
SIGNAL
CIRCUIT
KEYBOARD
MATRiX
18 X 8 LINES
( ) CONN.
1<8 DRIVE
()'17
+5V_
J4
KEYBOARD
CABLE
CONNECTOR
Figure 7-4
LK20l Keyboard Block Diagram
7-4