RS-485 specifies that the receiver output state should be logic high for differential input voltages of V_AB ≥ +200 mV and logic low for V_AB ≤ –200 mV. For input voltages in between these limits, a receiver’s output state is not defined and can randomly assume high or low. Removing the uncertainty of random output states, modern transceiver designs include internal biasing circuits that put the receiver output into a defined state (typically high) in the absence of a valid input signal.

A loss of input signal can be caused by an pen circuit caused by a wire break or the unintentional disconnection of a transceiver from the bus. The AM26C32 has an internal circuit that ensures functionality during an idle bus.