3 Increasing noise immunity

Unbalanced or single-ended drivers and receivers are inadequate for accomplishing a robust data link over long distances, as they are susceptible to common-mode noise. An excellent method to eliminate common-mode noise in a synchronous, full-duplex interface such as an SPI is the use of RS-422 differential driver and receiver circuits in combination with twisted-pair cable.

Because the conductors of twisted-pair cable are closely electrically coupled, external noise induced equally into both conductors appears as common-mode noise at the receiver input. Although differential receivers are sensitive to signal differences, they are immune to common-mode signals. The receiver therefore rejects common-mode noise, and signal integrity is maintained.

Another benefit of close electric coupling is that the currents in the two conductors create magnetic fields that cancel each other. The initial transversal electromagnetic (TEM) waves of the two conductors are therefore largely reduced to electric fields that cannot radiate into the environment (see Figure 4). Only the far smaller fringing fields outside the conductor loop can radiate, thus yielding much lower electromagnetic interference (EMI).