RS-422 and RS-485 receiver input pins all contain an attenuation network to translate input signals at the -7V and +12V extremes to voltage levels within the common mode range of the comparator stage. Receivers powered by 3V require a larger attenuation factor to bring a +12V signal below the comparator’s Vcc level. The resistor values in the attenuation network determine the unit load rating of the inputs, so the minimum resistor values are around 12 kΩ. For a 1/8-unit load rating, the two resistors of the attenuation network must sum to greater than 96 kΩ over the full process range. The large value pull-up resistor on the A input provides the failsafe open function. It is sized to bias the A input a few hundred mV above the B input when they are both floating, which guarantees a high level receiver output. High ESD transceivers utilize a bidirectional ESD network that is referenced only to ground. The breakdown voltages are asymmetrical to provide maximum protection without encroaching on the -7V to +12V input voltage range.