• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 15 subscribers
  • Views 917 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ISO3082: Isolated Master and Isolated Slave using RS485

Norman Oliva
Prodigy 210 points
Part Number: ISO3082
Other Parts Discussed in Thread: ISO1500, ISO1410

Hello,

Can you advise if below signal from RS485 A/B pins ( 12 and13 ) using ISO3082 is normal. Said ISO3082 part is already communicating with my slave device but I just noticed that the waveform does not have thesame amplitude.

Is this normal and is there any concern on this one? If this is a concern, any idea how to fix this?

Thanks,

Norman

  • 0
    TI__Intellectual 1960 points

    Hi Norman:

    Thanks for posting your question on E2E.

    Unfortunately, your waveforms didn't get attached to your post. Could you please add the image to this thread again? You may follow these steps when attaching an image.

    In addition, please be sure to include following information.

    1. How many nodes are on the RS-485 bus?
    2. How is the RS-485 bus terminated?
    3. Are the waveforms probed at the ISO3082 A/B pins?

    Lastly, you may consider our latest ISO1410 (P2P with ISO3082) and/or ISO1500 (smaller package option) for your newer designs.

    Regards,

    Dushmantha

  • 0 in reply to Dushmantha Rajapaksha
    Prodigy 210 points

    Hi  Dushmantha,

    Please see below:

    1. How many nodes are on the RS-485 bus? = Only one slave is connected to ISO3082.
    2. How is the RS-485 bus terminated? I have 121R on slave side and 121R on the IS03082 side
    3. Are the waveforms probed at the ISO3082 A/B pins?, i just probed at the middle of the 1fooot twisted pair wire.

  • 0 in reply to Norman Oliva
    Prodigy 210 points

    Resend to attach waveform. eattaching waveform, please let me know if it did not go through again 

  • 0 in reply to Norman Oliva
    TI__Intellectual 1960 points

    Hi Norman:

    Thanks for reattaching the waveforms and providing the additional details.

    Yes, this (A-B) output amplitude is lower than expected. I'd expect the amplitude to be about +/-2.3V with 60Ohm effective termination and under nominal conditions (Vcc2=5V and room temperature).  

    In order to debug this, can you please share the A and B output single ended waveforms (probed with respect to GND2)?

    Can you also share the following information?

    • Have you observed this reduced amplitude on multiple devices?
    • What's the Vcc2 value during these measurements?
    • Can you please share the schematic?

    Thanks,

    Dushmantha

  • 0 in reply to Dushmantha Rajapaksha
    Prodigy 210 points

    Hi Dushmantha, 

    I did some more verifications on my side and found out the Vcc2 should be 5V and I was using only 3.3V. It did improve the signal levels but it is not exactly  thesame amplitude. I tried to play around with R27 ( IS03082 A/B ) termination and found that 470R makes the level more equal but in this case the termination is now imbalance. I did check/compare the amplitude at both A/B end pins of both ISO3082 and MAX3079EESD+T devices at thesame time and they are exactly thesame. Not sure what can be the concern if i leave 120R/470R termination with equal amplitude on both A/B ends against 120R/120R termination with 200mV amplitude difference. In my application, I only use one slave device nothing in parallel.

    Note: there is an error on R26 and R28 connections but in this case I just remove them for the mean time including making R19/R20 change from 10R to 0R.

  • 0 in reply to Norman Oliva
    TI__Intellectual 1960 points

    Hi Norman:

    It's great to hear that the A-B signal amplitude is within expectations after changing the Vcc2 supply to 5V. Please note our ISO3082 Vcc2 should be between 4.5V and 5.5V for the correct operation.

    200mV delta between the two transceivers are expected due to the device differences. You don't have to match the primary and secondary side amplitudes tor the correct operation of your system. RS-485 is a robust physical layer. All RS-485 transmitters will output >1.5V output swing with 60-Ohm termination. Receiver inputs can recognize signals as low as 200mV over a common mode range of -7V to +12V.

    I'd recommend that you keep 120R/120R termination for the best signal integrity.

    Regards,

    Dushmantha

  • 0 in reply to Dushmantha Rajapaksha
    Prodigy 210 points

    Thank You Dushmantha for your responses.