Marantz 5220 User Manual download pdf (Page 59)

tal

electronic

circuits as

shown in Fig.

2. The components in

this

circuit

can

be

bought

from

almost any

"blister

package"

rack.

The three

7490

decade

counters represent the

output

stages

of a digital instrument.

Although

only

three

decades are shown, the circuit

can

be easily expanded

to six decades

simply by adding

AND

gates and

diodes or to

nine decades

by duplicat-

ing IC3.

In Fig. 2,

IC1 and

IC2

act

as

the rot-

ary switches

of Fig.l. They are

driven

by /C1,

which

gets

a clock

pulse to

determine the

multiplex

rate.

Each

output of IC2 turns on a single

digit of

the multi -digit

readout by shunting

a

cathode

to

ground

through

IC3.

Simultaneously,

the output

signal

from IC2 turns on

its set of

AND gates

to transfer

the associated

decade

counter's

BCD

output

to

the common

BCD -to-

seven

-segment decoder

/C4.

In

the digital

readout block,

all

the

similar segments

are

connected

in

parallel

so

that (for example)

if seg-

ment A

is driven, all

A

segments are

driven.

However,

the digit driving

cir-

cuit

has

activated

only one digit so

only the

A

segment of

that digit will

glow. The clock pulses then cause IC2

to switch

to its next output so

that the

second digit

is

activated.

If

the clock

rate

to

IC1 is

high

enough

(above 200

Hz),

each

digit is strobed

rapidly so

that the display

appears to be

motion-

less. When

the

states of the

7490 IC's

change,

the

readouts change accord-

ingly.

As

a

rule, the output of

a

TTL gate

is

intended

to drive

the input of another

gate.

If

the outputs are

tied together,

it

is possible that,

when one gate

has

a

high

output, one of the

others

may

have

a

low output.

The latter

will then

try to "sink

"the

output currents.

This

causes one

gate

to

overheat and pos-

sibly

malfunction.The diodes on

the

outputs of the

AND

gates

in Fig. 2 pre-

vent

this problem

by

isolating the out-

puts.

ResistorsRl

through

R4 provide

a

return

path

to ground to

insure that

the inputs of IC4

are

low if

no

positive

input

is

present.

The clock signal

for

/C1

can be ob-

tained from

any

source of pulses

whose

frequency is

greater

than

200

Hz.

If more than three decades are re-

quired, select an

output

from

IC2 that

is

one

digit higher than the

number of

digits

to

be displayed.

Then

invert this

signal and apply

it

to

the

clear

input

of

/C1. Expand the

circuit

by

coupling

the diodes to the BCD inputs of IC4.

Each /C3

can

handle

six digits.

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