Tandy PC-6 - Internal Circuits

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Buzzer Control Circuit

There

are

3 ways

to

operate the buzzer:

Key

input,

BEEP

0, and

BEEP

1

8

Mode

Frequency

Time

BEEPO

0.9 kHz

110

msec.

BEEP 1

4.55 kHz

110

msec.

Key

Input

4.55 kHz

11

msec.

A

82 t----=1..-

..,.,....,

81

----

M864H193

A

---.fLJLJL

8LSlSlJ

Interrupt

Circuit

•

Interrupt

circuit

for

the clock pulse when

VDD2

drops.

1.

When

the power

is

on,

VDD2

is

"L"

level.

2.

PG2

through the IC and 26th pin

of

the

CPU

receive

PG2.

3.

When

VDD2

drops, the

output

of

the IC becomes

"GND"

level and the

CPU

does

not

receive

PG2.

PG1

HD61747825

f---

PG2

VDD2

•

Interrupt

circuit

for

the voltage

of

the Gate Array and options when

VDD2

and

SW

drop.

1.

When

the power

is

on,

VDD2

and

SW

is

"L"

level.

2.

VDD2

for

Gate Array and

VDD4

is

also

"L"

level.

3.

When

VDD2

drops,

VDD2

for

Gate Array

and

VDD4

becomes

"GND"

level and stops the op-

ration

of

the RAM and options.

sw~

VDD2(from

IC 4)

VDD4

VDD2(from

IC 2)

Keyboard

HD61747B25 controls the upper keyboard.

Ks2

Ks3'VKs8

Upper

HD6174782

Ks9, Ks10

~

KeYboard

HD61747B20 controls the lower keyboard.

Km1'VKm4

Km5'VKm10

Lower

HD6174782

Keyboard

Km11, Km12

-6-