RS-422A vs RS-232C distance and speed

RS-422A uses balanced differential signalling on twisted pairs, whereas RS-232C and RS-423A use single-ended (unbalanced) signalling referred to a common ground. This is the reason for its longer distance and higher speed.

Reasons

1. Noise immunity: each signal travels on two wires with opposite polarity (A and B). The receiver is a differential amplifier, so noise picked up equally on both wires (common-mode noise) cancels. In RS-232C the noise adds directly to the single signal wire.
2. Ground potential difference: in RS-232C/423A the signal is measured against a shared ground, so a ground voltage difference between distant devices causes errors. In RS-422A the receiver senses only the difference between the two wires, so ground shifts (up to several volts) are tolerated.
3. Low crosstalk and radiation: the currents in the pair are equal and opposite, and the twisting reduces coupling.
4. Terminated transmission line: a 100 ohm terminator avoids reflections, so fast edges can be used on long cables. RS-232C lines are unterminated and the cable capacitance is limited to 2500 pF, which limits the length to about 15 m.
5. Sensitive receiver: it detects a difference as small as 200 mV, so the signal can weaken over a long cable and still be received.
6. Strong line drivers with low output impedance and high drive current.