MM74HC942N datasheet(4/8 Pages) NSC

4 / 8 page

Description of Pin Functions

Name

Function

DSI

Driver Summing Input This may be used to

transmit externally generated tones such as

dual tone multifrequency (DTMF) dialing sig-

nals

ALB

Analog Loop Back A logic high on this pin

causes the modulator output to be connect-

ed to the demodulator input so that data is

looped back through the entire chip This is

used as a chip self test If ALB and SQT are

simultaneously held high the chip powers

down

3CD

Carrier Detect This pin goes to a logic low

when carrier is sensed by the carrier detect

circuit

CDT

Carrier Detect Timing A capacitor on this

pin sets the time interval that the carrier

must be present before the CD goes low

RXD

Received Data This is the data output pin

6VCC

Positive Supply Pin A a5V supply is recom-

mended

CDA

Carrier Detect Adjust This is used for ad-

justment of the carrier detect threshold Car-

rier detect hysteresis is set at 3 dB

XTALD

Crystal Drive XTALD and XTALS connect

to a 35795 MHz crystal to generate a crys-

tal locked clock for the chip If an external

circuit requires this clock XTALD should be

sensed If a suitable clock is already avail-

able in the system XTALD can be driven

XTALS

Crystal Sense Refer to Pin 8 for details

FTLC

Filter TestLimiter Capacitor This is con-

nected to a high impedance output of the

receive filter It may thus be used to evalu-

Name

Function

ate filter performance This pin may also be

driven to evaluate the demodulator RXA1

and RXA2 must be grounded during this

test

For normal modem operation FTLC is AC

grounded via a 01 mF bypass capacitor

TXD

Transmitted Data This is the data input

VBB

Negative Supply The recommended supply

is b5V

OriginateAnswer mode select When logic

high this pin selects the originate mode of

operation

SQT

Squelch Transmitter This disables the mod-

ulator when held high The EXI input re-

mains active If SQT and ALB are simulta-

neously held high the chip powers down

RXA2

Receive Analog

2 RXA2 and RXA1 are

analog inputs When connected as recom-

mended they produce a 600X hybrid

RXA1

Receive Analog

1 See RXA2 for details

TXA

Transmit Analog This is the output of the

line driver

EXI

External Input This is a high impedance in-

put to the line driver This input may be used

to transmit externally generated tones

When not used for this purpose it should be

grounded

GND

Ground This defines the chip 0V

TLA

Transmit Level Adjust A resistor from this

pin to VCC sets the transmit level

Functional Description

INTRODUCTION

A modem is a device for transmitting and receiving serial

data over a narrow bandwidth communication channel The

MM74HC942 uses frequency shift keying (FSK) of an audio

frequency tone The tone may be transmitted over the

switched telephone network and other voice grade chan-

nels The MM74HC942 is also capable of demodulating

FSK signals By suitable tone allocation and considerable

signal processing the MM74HC942 is capable of transmit-

ting and receiving data simultaneously

The tone allocation by the MM74HC942 and other Bell 103

compatible modems is shown in Table I The terms ‘‘origi-

nate’’ and ‘‘answer’’ which define the frequency allocation

come from use with telephones The modem on the end of

the line which initiates the call is called the originate mo-

dem The other modem is the answer modem

TABLE I BELL 103 Allocation

Data

Originate Modem

Answer Modem

Transmit

Receive

Transmit

Receive

Space

1070Hz

2025Hz

1070Hz

Mark

1270Hz

2225Hz

1270Hz

THE LINE INTERFACE

The line interface section performs two to four wire conver-

sion and provides impedance matching between the mo-

dem and the phone line

THE LINE DRIVER

The line driver is a power amplifier for driving the line If the

modem is operating as an originate modem the second har-

monics of the transmitted tones fall close to the frequencies

of the received tones and degrade the received signal to

noise ratio (SNR) The line driver must thus produce low

second harmonic distortion

THE HYBRID

The voltage on the telephone line is the sum of the transmit-

ted and received signals The hybrid subtracts the transmit-

ted voltage from the voltage on the telephone line If the

telephone line was matched to the hybrid impedance the

output of the hybrid would be only the received signal This

rarely happens because telephone line characteristic im-

pedances vary considerably The hybrid output is thus a

mixture of transmitted and received signals

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