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TIDA-010026: How to evaluate communication quality with general oscilloscope

Hui Chen
Intellectual 770 points
Part Number: TIDA-010026
Other Parts Discussed in Thread: TIDA-01630

HI！Hello everyone

We use SN65hvd3088ed  to realize RS485 communication with tamakawa encoder.Communication baud rate is 2.5M.We need to evaluate the impact of cable length on communication quality. We hope that through the test, we can know the maximum communication distance of our system using the current cable.Tida-010026 describes that communication quality can be evaluate the quality of atransmitted signal through Eye Diagram, but our oscilloscope does not have  Eye Diagram function. Can we analyze it through VA and VB waveforms?

We built the test system, as shown in the figure below.

1.Signal waveform of 10 meter cable at encoder end

2.Signal waveform of 50m cable at encoder end

The purple line in the figure below is  VA-VB

3.Signal waveform of 50m cable at SN65hvd3088

The purple line in the figure below is  VA-VB

4.Summary

The signal waveform is seriously distorted when the cable is long.

The waveform  is different   at the encoder or at chip 3088.

VA-VB is more than 200mV

How can we evaluate the stability margin of communication.

  • 0
    TI__Expert 7850 points

    From the single ended signals you are showing on your scope plot you can see the Tamagawa encoder termination option, this is causing the distortion.

     

    Here you can see how to change the termination and what effect this has on the signal. This is documented in the TI design TIDA-01630 section 3.2.2.2

     

    Without infinite persistence option on your oscilloscope, making an assessment of the data integrity of your signal is difficult close to impossible.

    You can do some snap shot measurements, getting a good assumption of the signal, but using it for defining the quality of your signal I would not recommend.

     

    Many new scopes have this feature, one example is see link.