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RS485 Transceiver with noise immunity

Other Parts Discussed in Thread: SN65HVD75

Hi,

I'm working in the train sector and I'm searching for a new RS-485 transceiver. It would be used in a CAN-based bus called MVB (Multifunction Vehicle Bus). The standard (IEC 61375-3) states, that the receivers in the MVB-Bus need a noise immunity of +/- 100mV. In RS-485 transceiver datasheets the values Vit+ and Vit- (Positive/Negative receiver differential input voltage) are specified. To fulfill the standard either Vit+ has to be higher than +100mV or Vit- has to be lower than -100mV. In most cases there is no Min/Max value specified. Also the Hysteresis must be between 50mv and 200mV.

The closest device I found would be the SN65HVD75 . It would achieve a minimal noise immunity of +/- 70mV with a 50mV hysteresis.

Are there any other devices that would fit my requirements? (Maybe in development …) Also it would be nice if there is an isolated device so I don't need to add a separate isolator (Closest I found: ISO1176).

Best regards

Roger

  • Hi Roger,

    I'm not familiar with that standard, so I would like to see if I have access to a copy or can obtain one to better understand its requirements.

    The VIT(+) and VIT(-) threshold specifications don't really tell you anything about noise immunity.  They only address differential voltages above and below those thresholds, but don't specify the minimum distance between those two thresholds, ie the noise immunity.  That's the purpose of the Vhys specification, as you mention.

    It seems as though you're specifying noise immunity only during the failsafe condition, is that correct?  Otherwise, during operation, I would say the minimum Vhys of the SN65HVD75 only ensures immunity to noise less than 50mVpp.  If the requirement is only during a failsafe condition, it could be satisfied with external biasing.  If it is during operation, then you would require a minimum Vhys of 200mV, which I am not aware of an RS-485 receiver that meets that requirement.

    If you can clarify how and under what conditions the noise immunity is specified, it would help to reach a more definitive conclusion.

  • Hi Bart,


    Thank you for your response. The noise immunity has to be during the normal operation. But it won't need a Vhys of 200mV.

     

    This picture at page 15 in the datasheet of SN65HVD75 explains it. I would need a Vnoise-max of 200mVpp. So VIT+  and/or Vhys have to change. For example if the max value uf VIT+ would change form -20mV to -40mV  and Vhys would change from 50mV to 60mV it would be enough. I hope I was able to clarify the conditions.

     

    Regards,

     

    Roger

  • Roger,

    Figure 20 depicts noise immunity for the case VID = 0V, which it would be during a bus idle or bus short condition.  During operation, VID may be any value.  Consider the case during a transition from high to low or low to high where VID = -45mV, or halfway between -20mV and -70mV.  In this case, differential noise of the magnitude depicted in Figure 20 (or any noise greater than 50mVpp) could/would cause glitches in the output.  This is why I say that glitch-free operation in the presence of +/-100mV noise would require a hysteresis window of 200mV. 

  • Bart,

    I now see the missunderstanding. If a Transmitter sends a signal over the bus to the receiver and because of the loss of the cable the amplitude at the receiver is below 100mV, the signal should not be detectet. So for the receiver it would look like a bus idle or bus short condition, altough technically it isn't.

    Regards,

    Roger

  • Hello both of you,

    meanwhile I've got an information from Roger's colleague Ragesh Nair that there is a standard for such RS-485 transceivers to be compliant: IEC61375.

    Does that bring us any further?

    Regards,

    Marno