Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 34)

and open

and 0.039- second

closed.

Each

100- millisecond (0.100-

second)

period

represents a

dial pulse, and

each

pulse

represents a part of a

num-

ber. The

number of pulses

con-

tinuous

train is the same

as the

num-

ber of

the digit dialed.

For example

six

pulses are generated

when the 6

key on the keypad

is pressed, and

pulses are

generated

when

the 0 (Op-

erator) key

pressed.

Most residential

telephones

in the

U.S.

still use

mechanical

pulse dial-

ers. Since

mechanical

elements are

much less

reliable than are

all -elec-

tronic devices,

this explains

why dial-

ing problems

are the

most frequently

encountered.

Repairing a

rotary dialer

is no easy

task.

Pulse rate and

dwell time are

determined

by spring

tension and

mechanical

spacing

inside the dialer.

In the case of

the pulse

rate, the

me-

chanical

governor uses

spring tension

to control

"fly- away"

weights used

to slow down

the

return of the dial.

Too

little

spring

tension

these

counterweights,

and

the dialer

pulses

too slowly;

too much, and

returns

too quickly.

Dwell

time is determined

the

gap between

the pulser contact

points

in relationship

to the cam angle.

It's

set

by a procedure

very

similar

to the

setting

of points

car and

is just as

tricky to perform.

Dwell

time can also be altered

both contact

resistance and

contact

tension,

in addition

to the point gap,

so check all

possibilities. Pulse

rate is

less

critical,

and

may

vary

by as

much

as 100 percent

(up to

21 pulses per se-

cond)

many exchanges.

Electronic

Pulse Dialers

VLSI (Very Large

Scale Integration)

IC circuits

and pushbutton

keypads

have made

the

pulse

dialer

more re-

liable.

In place of

the rotating cam

and pulser

contacts,

the new ICs

place digital

pulses directly on

the

telephone

line electronically.

There are

many different types

pushbutton

pulse

dialers, but all

function

the same.

A representative

LOW

GROUP

697

770

852 Hz

941 Hz

HIGH GROUP

1477 Hz

1336 Hz

1209 Hz

Fig. 7. A

block

diagram

of a typical

electronic pulse dialer.

block diagram

shown

in Fig.

7. The

keypad

logic changes

a 3 -by

-4

push-

button

matrix input

into a series

output

pulses

that corresponds

to the

numeral

key pressed.

The pulse

train

is timed

exactly

the same as

the

rotary

dial,

with 61 and

milliseconds

off

and

on times,

respectively.

least

750 milliseconds

delay

is in-

serted

between

pulse streams

(num-

bers)

to avoid

run

-on.

Most of us

can punch buttons

fast-

er than

the dialer can

output pulses,

though,

so the chip

contains a

mem-

ory

feature.

When a number

is en-

tered, it's stored

in a

memory register

inside the chip. As

the pulses are

out-

putted, the chip

recalls

these

num-

bers

from memory

in the same

se-

quence

which they

were entered.

This is what

makes possible

the auto-

matic- redial

function found

in many

modern telephones.

Since

the tele-

phone

number

last dialed

remains in

memory, a

single

keystroke can

recall

it.

Because of this,

most electronical-

ly pulsed phones

have this feature.

Very little can

wrong

with

electronic

pulse

dialer,

other

than

failure of

the chip.

Almost all

func-

tions are contained

within

the chip

it-

self -with the possible exception

the

power supply.

Digital dialers derive power

from

the 50 -volt telephone

line. In most

cases,

the

diodes

needed to steer the

source

voltage

aren't inside the chip.

They're found as discrete compon-

ents on the printed- circuit board,

usually

within easy reach of the

hookswitch.

Should one

of the diodes

fail,

strange

things can -and do-

occur.

There are

four diodes, usually

of the

1N4000 series, arranged

as a full

wave

bridge

rectifier.

This

configura-

tion doesn't

change ac

into dc;

rather,

steers

the positive and

nega-

tive lines so that

the telephone

isn't

polarity sensitive.

if your auto-

matic pluse

dialer

isn't working, the

first

thing

to check are the diodes.

Touch

Tone Dialing

Touch Tone

signaling system

that's

rapidly

replacing the

rotary

dial and

holds the promise

of ex-

panded

services

for the

future. Tech-

nically defined

as Dual

-Tone Multi

Frequency (DTMF)

communications,

Touch Tone offers

increased speed

and the ability

to communicate

data.

Instead

of sending

groups of

pulses

to the central

office,

Touch

Tone transmits pairs

of tones

to the

central exchange,

where they're de-

ciphered

by special

receiving

equip-

ment. Because the

DTMF tones are

the audio

range, two tones are used to

distinguish

between actual

signaling

and

voice.

Tone signaling

is based on

internationally accepted standard

of frequencies that

includes four

tones of a

lower

and

three of

higher frequency. All digits are de-

rived

by combining

one

low group

frequency and one

high group fre-

quency, as

illustrated in Fig. 8.

DTMF tones are

sent

with

mini-

mum

duration

of 50

milliseconds and

interdigit time of

45 milliseconds.

The tone frequencies

must be ac-

curate

within 1.5

percent

their

stipulated

value under all conditions

temperature and

voltage

through-

MODERN

ELECTRONICS

/ April 1986

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You Saw

It In Modern Electronics

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