DEC VT220 - Page 141

,...-----r--+12V
8051
PORT 2
74LS05
INVERTERS
ON/OFF CONTROL
2KHZIU
BEEPER
Figure 7-8 Beeper Control Circuit
805'
PORT 2
74LS05
INVERTERS
A--r--+12V
HIGH
Figure 7-9
r--------
+ HIGH SIGr,AL FROM 8051 PROVIDES
PATH THROUGH LAST STAGE OF
OPEN COLLECTOR INVERTER TO TURN
ON LED
Indicator (LED) Control Circuit
7-15