DEC VT220 - Page 140

7.4.2
Audio Transducer Control Circuit
Figure 7-8 shows the audio transducer or beeper control circuit.
The beeper is driven by a transistor whose base is connected to a
2
KHz square wave from a 556 timer IC. This signal is biased by a
network of four type 74LS05 open collector inverters. The 8051
microprocessor controls all four inverters via the firmware. The
on/off inverter connects directly to the transistor base. When the
8051 puts a high on the on/off inverter input, its output goes low
and removes the 2 KHz square wave from the transistor base. This
cuts off the transistor and disables the beeper.
To turn on the beeper, the 8051 puts a low on the on/off inverter
input. Its output goes high and allows the 2 KHz signal to reach
the transistor base; this turns on the beeper. The firmware
generates a keyclick (on for 2 ms) or a bell tone (on for 125 ms) •
The
8051 sets up the three level control inverters by putting one
of eight binary combinations on the inverter inputs. All highs
give the softest sound and all lows give the loudest sound.
The firmware controls the keyclick
independently. The bell tone is sounded only
system control processor. The keyclick
disabled) under the following conditions.
• When a key is pressed
• When a metronome code is sent
and the bell tone
upon request from the
is sounded (unless
•
7.4.3
When a command to sound the keyclick is received from the
system control processor
Indicator (LED) Control Circuit
Figure 7-9 shows the LED indicator control circuit.
The control signal for each LED comes from PORT 2 of the 8051 to
the input of a type 74LS05 open collector inverter. The inverter
output goes to the LED cathode; its anode is connected to +5 V. A
separate +5 V source relieves the LEOs load on the main +5 V
supply.
A low signal from the 8051 drives the inverter output high, which
cuts off the LED. A high signal from the 8051 drives the inverter
output low. This provides a path to ground from the +5 V through
the LED. The LED then turns on.
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