SN65HVD72
SN65HVD75
SN65HVD78
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SLLSE11B –MARCH 2012–REVISED JUNE 2012
Receiver Failsafe
The differential receiver is “failsafe” to invalid bus states caused by:
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open bus conditions such as a disconnected connector
shorted bus conditions such as cable damage shorting the twisted-pair together, or
idle bus conditions that occur when no driver on the bus is actively driving
In any of these cases, the differential receiver will output a failsafe logic High state so that the output of the
receiver is not indeterminate.
Receiver failsafe is accomplished by offsetting the receiver thresholds so that the “input indeterminate” range
does not include zero volts differential. In order to comply with the RS-422 and RS-485 standards, the receiver
output must output a High when the differential input VID is more positive than +200 mV, and must output a Low
when the VID is more negative than -200 mV. The receiver parameters which determine the failsafe performance
are VIT+ and VIT-and VHYS. As seen in the Electrical Characteristics table, differential signals more negative than -
200 mV will always cause a Low receiver output. Similarly, differential signals more positive than +200 mV will
always cause a High receiver output.
When the differential input signal is close to zero, it will still be above the VIT+ threshold, and the receiver output
will be High. Only when the differential input is more negative than VIT-will the receiver output transition to a Low
state. So the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis
value VHYS (the separation between VIT+ and VIT-) as well as the value of VIT+
.
Signals which transition from positive to negative (or from negative to positive) will transition only once, ensuring
no spurious bits.
Low-Power Standby Mode
When both the driver and receiver are disabled (DE low and RE high) the device is in standby mode. If the
enable inputs are in this state for a brief time (e.g. less than 100 ns), the device does not enter standby mode.
This prevents inadvertently entering standby mode during driver/receiver enabling. Only when the enable inputs
are held in this state a sufficient duration (e.g. for 300 ns or more), the device is assured to be in standby mode.
In this low-power standby mode, most internal circuitry is powered down, and the supply current is typically less
than 100 nA. When either the driver or the receiver is re-enabled, the internal circuitry becomes active.
Copyright © 2012, Texas Instruments Incorporated
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