Data Communications Networking Devices - 4th Ed.pdf

1.14 THE PHYSICAL LAYER ________________________________________________________ 91Figure 1.47RS-422 cable distance plotted against signaling rateshift,source and load impedance of RS-232,which is approximately 5 k,isreduced to 100 by the use of differential signaling. Another bene®t of thissignaling method is the effect of noise on signal distortion. Since the presence ofnoise results in the same voltage being imposed on both wires,there is no change inthe voltage difference between the signal wires. Thus,the combination of lowersignaling levels and impedances coupled with voltage difference immunity to noisepermits differential signaling to drive longer cable distances at higher speeds. InFigure 1.47 you will ®nd a plot of cable distances against signaling rate for the RS-422 standard.For comparison purposes,the RS-232/V.24 cable distance is plotted against thesignaling rate in Figure 1.47. As indicated,RS-422 offers signi®cant advantagesover RS-232 with respect to both cabling distance and data signaling rate.RS-449RS-449 was introduced in 1977 as an eventual replacement for RS-232-C. Thisinterface speci®cation calls for the use of a 37-pin connector as well as an optional9-pin connector for devices using a secondary channel. Unlike RS-232,RS-449does not specify voltage levels. Two additional speci®cations known as RS-442-Aand RS-423 cover voltage levels for a speci®c range of data speeds. RS-442-A andits counterpart,ITU X.27 V.11),de®ne the voltage levels for data rates from20 kbps to 10 Mbps,while RS-423-A and its ITU counterpart,X.26 V.10),de®nethe voltage levels for data rates between 0 and 20 kbps.As previously mentioned,RS-442 as well as its ITU counterpart) de®nes theuse of differential balanced signaling. RS-422 is designed for twisted-pair telephonewire transmission ranging from 10 Mbps at distances up to 40 feet to100 kbps at distances up to 4000 feet. RS-423 de®nes the use of unbalancedsignaling similar to RS-232. This standard supports data rates ranging from100 kbps at distances up to 40 feet to 10 kbps at distances up to 200 feet.