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DS16F95QML: Data corrupted by probing differential pins but only on half of the chips purchased.

Carl Barcroft
Prodigy 40 points
Part Number: DS16F95QML
Other Parts Discussed in Thread: SN65HVD1787,

We initially designed our 485 communications around the SN65HVD1787P chip but for the finished product switched to the 5962-8961501PA. On half of the builds we noticed that when we probed a voltage supply on the board that the data from the RS-485 chip was corrupted. After some debugging we found that all we needed to do was touch a wire to pin 6 or 7, the differential lines, of the 485 chip and that would cause the data corruption. This only happens on half of the mil-spec chips that we've populated boards with and has not happened with the commercial chip mentioned above. Below is a diagram of our setup.

  • 0
    TI__Guru 74292 points

    Carl,

    An engineer has been notified of your question and will respond by the end of the business day CST tomorrow.

    Regards,

    Eric Hackett

  • 0
    TI__Mastermind 25705 points

    DS16F95QML generates differential RS-485 signal at A/B pins (pin 6 and 7). I guess the signal is no longer differential by touching a wire to either A or B. What's connected to the other end of the wire? Have you captured any waveforms? Please be aware that SN65HVD1787 has +30V/-70V fault voltage, while DS16F95QML is +15V/-10V. 

  • 0 in reply to Hao L
    Prodigy 40 points

    Nothing is connected to the wire. This fault is also chip dependent as it only happens to about half the chips we purchased. We've even swapped chips from boards that were working with boards that weren't and the boards would no longer exhibit this behavior. You don't have to tap on pins 6 or 7 to receive this issue. The issue also arises when any of the voltage supply rails are probed with a DMM lead. Attached is the requested waveform. First image is of DE and RE signals during normal operation, second image is DE and RE signals after we touch pin 6 or 7 with unconnected piece of bus wire and the third image is DE and RE signals after repeated touching of pin 6 or 7 with unconnected bus wire.

  • 0 in reply to Carl Barcroft
    TI__Mastermind 25705 points

    Carl,

    Thanks for the information. When tapping the Vcc rail, is there any voltage drop or Icc change? BTW, is a bypass capacitor (usually 0.1uF) populated on board? When tapping pin 6/7, what's the state of DI pin? Do you observe similar signal change at RO?