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National Semiconductor - Transmission Line Drivers and Receivers

Numéro de référence AN-214
Description Transmission Line Drivers and Receivers
Fabricant National Semiconductor 
Logo National Semiconductor 





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AN-214 fiche technique
Transmission Line Drivers
and Receivers for TIA/EIA
Standards RS-422 and
RS-423
Introduction
With the advent of the microprocessor, logic designs have
become both sophisticated and modular in concept. Fre-
quently the modules making up the system are very closely
coupled on a single printed circuit board or cardfile. In a
majority of these cases a standard bus transceiver will be
adequate. However because of the distributed intelligence
ability of the microprocessor, it is becoming common prac-
tice for the peripheral circuits to be physically separated from
the host processor with data communications being handled
over cables (e.g. plant environmental control or security
system). And often these cables are measured in hundreds
or thousands of feet as opposed to inches on a backplane. At
this point the component wavelengths of the digital signals
may become shorter than the electrical length of the cable
and consequently must be treated as transmission lines.
Further, these signals are exposed to electrical noise
sources which may require greater noise immunity than the
single chassis system.
It is the object of this application note to underscore the more
important design requirements for balanced and unbalanced
transmission lines, and to show that National’s DS1691
driver and DS78LS120 receiver meet or exceed all of those
requirements.
The Requirements
The requirements for transmission lines and noise immunity
have been adequately recognized by National Semiconduc-
National Semiconductor
Application Note 214
John Abbott
John Goldie
August 1993
tor’s application note AN-108 and TIA/EIA standards TIA/
EIA-422-B (balanced) and TIA/EIA-423-B (unbalanced). In
this application note the generic terms of RS-422 and
RS-423 will be used to represent the respective TIA/EIA
standards. A summary review of these notes will show that
the controlling factors in a voltage digital interface are:
1. The cable length
2. The data signaling rate
3. The characteristic of the interconnection cable
4. The rise time of the signal
RS-422 and RS-423 contain several useful guidelines rela-
tive to the choice of balanced circuits versus unbalanced
circuits. Figure 1 and Figure 2 are the digital interface for
balanced (1) and unbalanced (2) circuits.
Even though the unbalanced interface circuit is intended for
use at lower modulation rates than the balanced circuit, its
use is not recommended where the following conditions
exist:
1. The interconnecting cable is exposed to noise sources
which may cause a voltage sufficient to indicate a
change of binary state at the load.
2. It is necessary to minimize interference with other sig-
nals, such as data versus clock.
3. The interconnecting cable is too long electrically for
unbalanced operation (Figure 3).
http://www.DataSheet4U.net/
Legend:
Rt = Transmission line termination and/or receiver input impedance
VGROUND = Ground potential difference
A, B = Driver interface
A', B' = Load interface
C = Driver circuit ground
C' = Load circuit ground
FIGURE 1. Balanced Digital Interface Circuit
00585401
TRI-STATE® is a registered trademark of National Semiconductor Corp.
© 2002 National Semiconductor Corporation AN005854
www.national.com
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