AN-214

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 G...