9.4 Device Functional Modes

For the SN65HVD70, SN65HVD73, and SN65HVD76, when the driver enable pin, DE, is logic high, the differential outputs Y and Z follow the logic states at data input D. A logic high at D causes Y to turn high and Z to turn low. In this case the differential output voltage defined as V_OD = V_(Y) – V_(Z) is positive. When D is low, the output states reverse, Z turns high, Y becomes low, and V_OD is negative.

When DE is low, both outputs turn high-impedance. In this condition the logic state at D is irrelevant. The DE pin has an internal pulldown resistor to ground, thus when left open the driver is disabled (high-impedance) by default. The D pin has an internal pullup resistor to V_CC, thus, when left open while the driver is enabled, output Y turns high and Z turns low.

When the receiver enable pin, RE, is logic low, the receiver is enabled. When the differential input voltage defined as V_ID = V_(A) – V_(B) is positive and higher than the positive input threshold, V_IT+, the receiver output, R, turns high. When V_ID is negative and less than the negative and lower than the negative input threshold, V_IT–, the receiver output, R, turns low. If V_ID is between V_IT+ and V_IT– the output is indeterminate.

When RE is logic high or left open, the receiver output is high-impedance and the magnitude and polarity of VID are irrelevant. Internal biasing of the receiver inputs causes the output to go failsafe-high when the transceiver is disconnected from the bus (open-circuit), the bus lines are shorted (short-circuit), or the bus is not actively driven (idle bus).

For the SN65HVD71, HVD74, and HVD77, the driver and receiver are fully enabled, thus the differential outputs Y and Z follow the logic states at data input D at all times. A logic high at D causes Y to turn high and Z to turn low. In this case the differential output voltage defined as V_OD = V_(Y) – V_(Z) is positive. When D is low, the output states reverse, Z turns high, Y becomes low, and V_OD is negative. The D pin has an internal pullup resistor to V_CC, thus, when left open while the driver is enabled, output Y turns high and Z turns low.

When the differential input voltage defined as V_ID = V_(A) – V_(B) is positive and higher than the positive input threshold, V_IT+, the receiver output, R, turns high. When V_ID is negative and less than the negative input threshold, V_IT–, the receiver output, R, turns low. If V_ID is between V_IT+ and V_IT– the output is indeterminate. Internal biasing of the receiver inputs causes the output to go failsafe-high when the transceiver is disconnected from the bus (open-circuit), the bus lines are shorted (short-circuit), or the bus is not actively driven (idle bus).