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DS26C32AM: Flexible RS-422/TTL input termination scheme

Craig Javid
Prodigy 250 points

Part Number: DS26C32AM

Dear forum:

 

Please see the attached sketch which is the reference for my questions.  I am using a DS26C32 RS-422 differential receiver in an application in which I desire to support both RS-422 inputs and TTL inputs.  Circuit 1 shows the classic differential DC termination.  This works well for RS-422, has higher DC dissipation and would not support TTL inputs.  Circuit 2 uses an AC differential termination and works OK with zero DC dissipation but would not support TTL very well.

 

In my third approach,  I use a split AC termination that looks to the transmitter like nearly 120 ohms for transitions and provides some filtering of noise.  The #A circuit is for RS-422 with A and B inputs while the 3B circuit uses the 10k/6.8k bias string to set the inverting input of the receiver near 2V,  A TTL input applied to the positive input with respect to ground see some input filtering but not a low value DC termination.  In my view this approach should work well in both applications. 

 

Are there any potential problems with my circuit 3 approach for RS-422 and TTL support?

 Thanks

interface options.pdf

  • 0
    TI__Mastermind 25705 points

    Craig,

    Your 3rd approach looks good to me except I think you might want to put 62Ohm resistor in serial with 2.2nF capacitor, just like circuit 2. Otherwise the signal at the receiver input will be attenuated by low pass filter. It is a good idea to use resistor divider to generate a bias for TTL. You can refer to this application note for similar design (http://www.ti.com/lit/an/snla041a/snla041a.pdf

  • 0 in reply to Hao L
    Prodigy 250 points
    HI Hao:

    Thank you for your feedback. I was aware that I was adding a low pass filter for the inputs with the RC filter at about 1.1MHz. I guess tose values are a bit too aggressive so I will change as you describe to use a series R-C from each receiver input to ground. Do you think the load of 62 ohms and 2.2nf on the positive input will cause a significant problem for the case where a TTL signal is driving the line? That was a concern of mine in either configuration. If the AC impedance is too low during the TTL edges, perhaps I could increase the series R somewhat maybe from 62 ohms to 100 ohms or so. That would adversely affect the termination for the RS-422 signal but maybe I need to compromise?

    Thanks,

    Craig
  • 0 in reply to Craig Javid
    TI__Mastermind 25705 points
    Craig,

    I'm not worried too much about TTL signal. Since it's slow, RC filter should not limit the bandwidth. But with current configuration, in RS422 communication, the differential input voltage of the receiver is not generated on 62Ohm resistors but on 2.2nF capacitor, whose equivalent impedance is about 72Ohm at 2Mbps data rate. I'm not sure you will get enough voltage difference. Hope this makes sense to you. Please let me know if you have any questions. Regards,
  • 0 in reply to Hao L
    Prodigy 250 points

    Hi Hao:

    I understand.  Please see my revised circuit per your recommendation.revised interface.pdf

  • 0 in reply to Craig Javid
    TI__Mastermind 25705 points

    Craig,

    The schematic looks good to me. I hope it work for you. Please let me know if you have any questions.

    Regards,