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参数资料
| 型号： | MM74HC942J |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 调制器/解调器 |
| 英文描述： | MM74HC942 is a full duplex low speed modem |
| 中文描述： | 0.3 kbps DATA, MODEM, CDIP20 |
| 封装： | DIP-20 |
| 文件页数： | 4/8页 |
| 文件大小： | 146K |
| 代理商： | MM74HC942J |
Description of Pin Functions
Pin
No.
Name
Function
1
DSI
Driver Summing Input: This may be used to
transmit externally generated tones such as
dual tone multifrequency (DTMF) dialing sig-
nals.
2
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.
3
CD
Carrier Detect: This pin goes to a logic low
when carrier is sensed by the carrier detect
circuit.
4
CDT
Carrier Detect Timing: A capacitor on this
pin sets the time interval that the carrier
must be present before the CD goes low.
5
RXD
Received Data: This is the data output pin.
Positive Supply Pin: A
a
5V supply is recom-
mended.
6
V
CC
7
CDA
Carrier Detect Adjust: This is used for ad-
justment of the carrier detect threshold. Car-
rier detect hysteresis is set at 3 dB.
8
XTALD
Crystal Drive: XTALD and XTALS connect
to a 3.5795 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.
9
XTALS
Crystal Sense: Refer to Pin 8 for details.
10
FTLC
Filter Test/Limiter Capacitor: This is con-
nected to a high impedance output of the
receive filter. It may thus be used to evalu-
Pin
No.
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 0.1
m
F bypass capacitor.
Transmitted Data: This is the data input.
11
TXD
12
V
BB
Negative Supply: The recommended supply
is
b
5V.
Originate/Answer mode select: When logic
high this pin selects the originate mode of
operation.
13
O/A
14
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.
Receive Analog
Y
2: RXA2 and RXA1 are
analog inputs. When connected as recom-
mended they produce a 600
X
hybrid.
Receive Analog
Y
1: See RXA2 for details.
15
RXA2
16
RXA1
17
TXA
Transmit Analog: This is the output of the
line driver.
18
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.
19
GND
Ground: This defines the chip 0V.
20
TLA
Transmit Level Adjust: A resistor from this
pin to V
CC
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
2025Hz
1070Hz
Mark
1270Hz
2225Hz
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.
4
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