When the driver enable pin, DE, is logic high, the differential outputs A and B follow the logic states at data input D. A logic high at D causes A to turn high and B to turn low. In this case the differential output voltage defined as VOD = VA – VB is positive. When D is low, the output states reverse, B turns high, A becomes low, and VOD 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 pull-down resistor to ground, thus when left open the driver is disabled (high-impedance) by default. The D pin has an internal pull-up resistor to VCC, thus, when left open while the driver is enabled, output A turns high and B turns low.
|H||H||H||L||Actively drive bus High|
|L||H||L||H||Actively drive bus Low|
|X||OPEN||Z||Z||Driver disabled by default|
|OPEN||H||H||L||Actively drive bus High by default|
When the receiver enable pin, RE, is logic low, the receiver is enabled. When the differential input voltage defined as VID = VA – VB is positive and higher than the positive input threshold, VIT+, the receiver output, R, turns high. When VID is negative and lower than the negative input threshold, VIT–, the receiver output, R, turns low. If VID is between VIT+ and VIT– 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).
|DIFFERENTIAL INPUT||ENABLE (1)||OUTPUT(1)||FUNCTION|
|VID = VA – VB||RE||R|
|VIT+ < VID||L||H||Receive valid bus High|
|VIT– < VID < VIT+||L||?||Indeterminate bus state|
|VID < VIT–||L||L||Receive valid bus Low|
|X||OPEN||Z||Receiver disabled by default|
|Open-circuit bus||L||H||Fail-safe high output|
|Short-circuit bus||L||H||Fail-safe high output|
|Idle (terminated) bus||L||H||Fail-safe high output|