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RS485 Fail safe and fault protection function

Shu-Cheng LIN
Guru 11785 points

Hi Sir,

Sorry to bother you.

We would like use TI RS 485
There had a lots solution.

We confuse at some Electrical Characteristics
May i know what is fault protection function??

Also there have some IC had Fail safe function. What is Fail safe??

From spec, there have Idle, Open, Short could choose. What different between them??

Thanks for your reply

  • TI__Guru 62920 points
    Hi Shu-Cheng,

    The terminology used to describe these features can be confusing, so I will try to help clarify.

    Fault protection generally means that the transceiver is not damaged if its RS-485 bus pins are shorted to a power supply. From a device perspective this translates to a wide absolute maximum range on the bus pins (pins A and B for a half-duplex transceiver).

    Failsafe means that the receiver output is in a known state when a fault condition occurs that causes the differential signal amplitude to be 0 V. Examples of these faults would be if the bus is idle (undriven), if the cable is disconnected, or if the two lines of the differential pair are shorted together. This function is usually implemented by shifting the receiver input thresholds (VIT+ and VIT-) so that they are both somewhere between 0 V and -200 mV. This means that a 0 V input would be interpreted as a logic "high." (If instead the receiver thresholds were close to 0 V, a loss of input signal resulting in a 0 V input could result in an indeterminate receiver output level.)

    I hope this is all clear; please let me know if you have any other questions.

    Regards,
    Max