Data Communications Networking Devices - 4th Ed.pdf

90 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSRS-232/V.24 is limited to a maximum data rate of 19.2 kbps at a cablingdistance of 50 feet. In actuality,speeds below 19.2 kbps allow greater transmissiondistances while for cable lengths of only a few feet a data rate over 100 kbpsbecomes possible.Differential signalingOver long cabling distances the cumulative cable capacitance and resistance combineto cause a signi®cant amount of signal distortion. At some cabling distance,which decreases in an inverse relationship to the data rate,the signal cannot berecognized. Thus,to overcome the cabling distance and speed limitations associatedwith RS-232 a different method of signaling was devised. This signalingtechnique,known as differential signaling,results in information being conveyedby the relative voltage levels in two wires. Instead of using one driver to produce alarge voltage swing as RS-232 does,differential signaling uses two drivers to split asignal into two parts.Figure 1.46 illustrates differential signaling as speci®ed by the RS-422 interfacestandard. To transmit a logical `1' the A driver output is driven more positivewhile the B driver output is more negative. Similarly,to transmit a logical `0' the Adriver output is driven more negative while the B driver output is driven morepositive. At the receiver a comparator is used to examine the relative voltage levelson the two signal wires.With the use of two wires,RS-449 speci®es a mark or space by the differencebetween the voltages on the two wires. This difference is only 0.4 V under RS-422,whereas it is 6 V ‡ 3 V and 3 V) under RS-232/V.24. Thus,if the differencesignal between the two wires is positive and greater than 0.2 V,the receiver willread a mark. Similarly,if the difference signal is negative and more negative than0.2 V,the receiver will read a space. In addition to requiring a lower voltageFigure 1.46 Differential signaling. The RS-422 speci®es balanced differential signalingsince the sum of the currents in the differential signaling wires is zero