AN-214| Application Note

Transmission Line Drivers and Receivers for TIA/EIA Standards RS-422 and RS-423

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. Frequently 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 practice 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.

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. 2. 3. 4. The cable length The data signaling rate The characteristic of the interconnection cable The rise time of the signal

The Requirements
The requirements for transmission lines and noise immunity have been adequately recognized by National Semiconduc-

RS-422 and RS-423 contain several useful guidelines relative 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 signals, such as data versus clock. 3. The interconnecting cable is too long electrically for unbalanced operation (Figure 3).

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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

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