SN65HVD37

SLLSE92 A

– OCTOBER 2011 – REVISED NOVEMBER 2011

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APPLICATION INFORMATION

RECEIVER FAILSAFE

The differential receiver is

“failsafe” to invalid bus states caused by:

•

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 outputs a failsafe logic High state, so that the output of the

receiver is not indeterminate.

In the HVD37, 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

of -120 mV and a minimum (most negative) value of -200 mV, so differential signals more negative than -200 mV

will always cause a Low receiver output. Similarly, differential signals more positive than 200 mV will always

-20 mV under any conditions of temperature, supply voltage, or common-mode offset.

state. So, the noise immunity of the receiver inputs during a bus fault condition includes the receiver hysteresis

For the HVD37, the typical noise immunity is about 120 mV, which is the negative noise level needed to exceed

MIN = 30 mV).

HOT-PLUGGING

These devices are designed to operate in

“hot swap” or “hot pluggable” applications. Key features for

hot-pluggable applications are power-up, power-down glitch free operation, default disabled input/output pins,

and receiver failsafe. An internal Power-On Reset circuit keeps the driver outputs in a high-impedance state until

the supply voltage has reached a level at which the device will reliably operate. This ensures that no spurious

transitions (glitches) will occur on the bus pin outputs as the power supply turns on or turns off.

As shown in the device FUNCTION TABLE, the ENABLE inputs have the feature of default disable on both the

driver enable and receiver enable. This ensures that the device will neither drive the bus nor report data on the R

pin until the associated controller actively drives the enable pins.

LOW POWER STANDBY MODE

As is customary with RS-485 devices, the receiver output is directly enabled/disabled by RE, and the driver

outputs are directly enabled/disabled by DE.

When both the driver and receiver are disabled, (DE=LO and RE=HI) the receiver differential comparator stage

enters a standby mode for reduced power.

When either the Driver or Receiver is enabled, the receiver differential comparator stage is enabled for fast

response to signal changes.

SPACER

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© 2011, Texas Instruments Incorporated

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