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PDF 74HC942 Data sheet ( Hoja de datos )
| Número de pieza | 74HC942 | |
| Descripción | full duplex low speed modem | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de 74HC942 (archivo pdf) en la parte inferior de esta página. Total 8 Páginas | ||
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January 1988
MM74HC942 300 Baud Modem
General Description
The MM74HC942 is a full duplex low speed modem It pro-
vides a 300 baud bidirectional serial interface for data com-
munication over telephone lines and other narrow band-
width channels It is Bell 103 compatible
The MM74HC942 utilizes advanced silicon-gate CMOS
technology Switched capacitor techniques are used to per-
form analog signal processing
www.DataSheet4U.com MODULATOR SECTION
The modulator contains a frequency synthesizer and a sine
wave synthesizer It produces a phase coherent frequency
shift keyed (FSK) output
LINE DRIVER AND HYBRID SECTION
The line driver and hybrid are designed to facilitate connec-
tion to a 600X phone line They can perform two-to-four-
wire conversion and drive the line at a maximum of 0 dBm
DEMODULATOR SECTION
The demodulator incorporates anti-aliasing filters a receive
filter limiter discriminator and carrier detect circuit The
nine pole receive filter provides 60 dB of transmitted tone
rejection The discriminator is fully balanced for stable
operation
Connection and Block Diagrams
Features
Y Drives 600X at 0 dBm
Y All filters on chip
Y Transmit level adjustment compatible with universal
service order code
Y TTL and CMOS compatible logic
Y All inputs protected against static damage
Y g5V supplies
Y Low power consumption
Y Full duplex answer or originate operation
Y Analog loopback for self test
Y Power down mode
Applications
Y Built-in low speed modems
Y Remote data collection
Y Radio telemetry
Y Credit verification
Y Stand-alone modems
Y Point-of-sale terminals
Y Tone signalling systems
Y Remote process control
TRI-STATE is a registered trademark of National Semiconductor Corp
Dual-In-Line Package
Top View
TL F 5348–1
Order Number MM54HC942 or
MM74HC942
C1995 National Semiconductor Corporation TL F 5348
TL F 5348 – 2
RRD-B30M105 Printed in U S A
1 page  
www.DataSheet4U.com
Functional Description (Continued)
THE DEMODULATOR SECTION
The Receive Filter
The demodulator recovers the data from the received sig-
nals The signal from the hybrid is a mixture of transmitted
signal received signals and noise The first stage of the
receive filter is an anti-alias filter which attenuates high fre-
quency noise before sampling occurs The signal then goes
to the second stage of the receive filter where the transmit-
ted tones and other noise are filtered from the received sig-
nal This is a switched capacitor nine-pole filter providing at
least 60 dB of transmitted tone rejection This also provides
high attenuation at 60 Hz a common noise component
The Discriminator
The first stage of the discriminator is a hard limiter The hard
limiter removes from the received signal any amplitude
modulation which may bias the demodulator toward a mark
or a space It compares the output of the receive filter to the
voltage on the 0 1 mF capacitor on the FTLC pin
The hard limiter output connects to two parallel bandpass
filters in the discriminator One filter is tuned to the mark
frequency and the other to the space frequency The out-
puts of these filters are rectified filtered and compared If
the output of the mark path exceeds the output of the space
path the RXD output goes high The opposite case sends
RXD low
The demodulator is implemented using precision switched
capacitor techniques The highly critical comparators in the
limiter and discriminator are auto-zeroed for low offset
Carrier Detector
The output of the discriminator is meaningful only if there is
sufficient carrier being received This is established in the
carrier detection circuit which measures the signal on the
line If this exceeds a certain level for a preset period (ad-
justable by the CDT pin) the CD output goes low indicating
that carrier is present Then the carrier detect threshold is
lowered by 3 dB This provides hysteresis ensuring the CD
output remains stable If carrier is lost CD goes high after
the preset delay and the threshold is increased by 3 dB
MODULATOR SECTION
The modulator consists of a frequency synthesizer and a
sine wave synthesizer The frequency produces one of four
tones depending on the O A and TXD pins The frequencies
are synthesized to high precision using a crystal oscillator
and variable dual modulus counter The counters used re-
spond quickly to data changes introducing negligible bit jit-
ter while maintaining phase coherence
The sine wave synthesizer uses switched capacitors to
‘‘look up’’ the voltages of the sine wave This sampled sig-
nal is then further processed by switched capacitor and
continuous filters to ensure the high spectral purity required
by FCC regulations
Applications Information
TRANSMIT LEVEL ADJUSTMENT
The transmitted power levels of Table II refer to the power
delivered to a 600X load from the external 600X source
impedance The voltage on the load is half the TXA voltage
This should be kept in mind when designing interface cir-
cuits which do not match the load and source impedances
The transmit level is programmable by placing a resistor
from TLA to VCC With a 5 5k resistor the line driver trans-
mits a maximum of b9 dBm Since most lines from a phone
installation to the exchange provide 3 dB of attenuation the
maximum level reaching the exchange will be b12 dBm
This is the maximum level permitted by most telephone
companies Thus with this programming the MM74HC942
will interface to most telephones This arrangement is called
the ‘‘permissive arrangement ’’ The disadvantage with the
permissive arrangement is that when the loss from a phone
to the exchange exceeds 3 dB no compensation is made
and SNR may be unnecessarily degraded
SNR can be maximized by adjusting the transmit level until
the level at the exchange reaches b12 dBm This must be
done with the cooperation of the telephone company The
programming resistor used is specific for a given installation
and is often included in the telephone jack at the installa-
tion The modem is thus programmable and can be used
with any jack correctly wired This arrangement is called the
universal registered jack arrangement and is possible with
the MM74HC942 The values of resistors required to pro-
gram the MM74HC942 follow the most common code in
use the universal service order code The required resistors
are given in Table II
TABLE II Universal Service Order Code Resistor Values
Line
Loss
(dB)
Transmit
Level
(dBm)
Programming
Resistor (RTLA)
(Ohms)
0 b12
1 b11
2 b10
3 b9
4 b8
5 b7
6 b6
7 b5
8 b4
9 b3
10 b2
11 b1
12 0
Open
19 800
9 200
5 490
3 610
2 520
1 780
1 240
866
562
336
150
0
CARRIER DETECT THRESHOLD ADJUSTMENT
The carrier detect threshold is directly proportional to the
voltage on CDA This pin is connected internally to a high
impedance source This source has a nominal Thevenin
equivalent voltage of 1 2V and output impedance of 100 kX
By forcing the voltage on CDA the carrier detect threshold
may be adjusted To find the voltage required for a given
threshold the following equation may be used
VCDA e 244 c VON
VCDA e 345 c VOFF
CARRIER DETECT TIMING ADJUSTMENT
CDT A capacitor on Pin 4 sets the time interval that the
carrier must be present before CD goes low It also
sets the time interval that carrier must be removed
before CD returns high The relevant timing equations
are
TCDL j 6 4cCCDT for CD going low
TCDH j 0 54cCCDT for CD going high
Where TCDL TCDH are in seconds and CCDT is in mF
5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet 74HC942.PDF ] | |
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