 It is the voltage difference between these two lines that define the mark and space, rather than, as in RS-232, the difference in voltage between a data line and a local ground.As the ground voltage can differ at either end of the cable, this required RS-232 to use large +5 and -5 voltages.Moving to dedicated return lines and always defining ground in reference to the sender allowed RS-422 to use 0.4 V, allowing it to run at much higher speeds.