• State Resolved
  • Locked Locked
  • Replies 13 replies
  • Answers 3 answers
  • Subscribers 34 subscribers
  • Views 742 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

TRS3221E: Layout trace length of TRS3221E

Lisa Xu
Intellectual 505 points

Part Number: TRS3221E

Tool/software:

Dear TI Team:

Now we have a big PCB size design which also use TRS3221E for RS232 tranceiver.

The signal between SOC and TRS3221E is UART TX/RX.

Per your experience, how long PCB trace can be allowed for T1 as below(between SOC and TRS3221E)?

Now on our board,T1 PCB length may be up to 50 inch. 

Best Regards

Lisa

  • 0
    TI__Guru 53952 points

    Hi Lisa,

    So we don't have a direct number for our RS-232 devices - generally we recommend relatively short traces. Since the output is going to be similar to a logic device from TI that would max out around 120mm (~4.7 inches) (source) - so at 50 inches (which is > 1m) you have a larger probability to see problems. This  is going to be an issue with all RS-232 devices - this isn't an issue with the TRS3221E specifically - reason being is that the assumption is that the transceiver is very close to controller and that the "length" of the RS-232 system is on the RS-232 bus. 

    With that in mind I see two possible solutions:

    1. The simplest solution is to move the transceiver closer to SOC - but I understand this may not be possible in your application - if not see point 2. 

    2. Adding a digital buffer on the TX and RX lines connected to controller - essentially allowing the signals to be driven by a stronger source. The source I linked above in the reply has some information on buffers that are built to drive 50 ohm transmission lines (so trace impedance would need to be controlled)

    I think there is a lot of risk to have a 50inch trace between transceiver and controller - so I'd suggest to move it closer or add buffers to ensure proper signal levels are maintained. 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson 

  • 0 in reply to Parker Dodson
    Intellectual 505 points

    Hi Parker:

    The source I linked above in the reply has some information on buffers that are built to drive 50 ohm transmission lines (so trace impedance would need to be controlled)

    ==>I cannot find this link, It seems no this link in the reply.

    Could you share again.or share the Buffer PN.Thanks

    Best Regards

    Lisa

  • 0 in reply to Lisa Xu
    Guru 320650 points

    All three links in the FAQ linked above work for me.

    All buffers with output-damping resistors are large multi-channel devices. In your application, it would much simpler to add source-termination resistors.

  • 0 in reply to Clemens Ladisch
    Intellectual 505 points

    Hi Parker, Clemens:

    I have found the "Source" link, As Clemens said the components linked are almost multi-channel device,

    Could I use buffer <SN74LVC1G125DCKT> for UART signal in this application?

    Best Regards

    Lisa

  • +1 in reply to Lisa Xu
    Guru 320650 points

    Yes, or any other '125 or '17 or '34 buffer.

    Please note that RS-232 is very slow, so you should not actually need a strong buffer. If any of the other devices has an output impedance smaller than about 50 Ω, you can add a source termination resistor.

  • +1 in reply to Lisa Xu
    TI__Guru 53952 points

    Hi Lisa,

    That buffer should be okay. 

    Output impedance is not smaller than 50 ohms on this device (it's typically 100 ohms, but can be as high as 600 ohms) - so add the buffer. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 505 points

    Hi Parker:

    Due to our PCB size is too big and the signal trace will be long, and we use PCB with ultra low Df,
    So our team plan to do SI(Signal Integrity) simulation,
    Our SI team ask for parameters< the tolerated insetion loss & the limiation of trace loss>.
    Could you provide these parameters for TRS3221E(SoC side) and other information you think which is helpful for our SI simulation.
    Thanks.
    Best Regards
    Lisa
  • 0 in reply to Lisa Xu
    Guru 320650 points

    There parameters are not specified for slow digital devices (and RS-232 is quite slow).

    What matters is that any ringing does not cross the VIL/VIH thresholds.

  • 0 in reply to Clemens Ladisch
    Intellectual 505 points

    Hi Clemens:

    What matters is that any ringing does not cross the VIL/VIH thresholds.

    ==>Can this ringing be simulated or not, Or How to simulated to get know the exact PCB trace length.

    or what information should I share to our SI team .

    Thanks

    Best Regards

    Lisa

  • 0 in reply to Lisa Xu
    Guru 320650 points

    The IBIS model has enough information to allow simulating this.

  • 0 in reply to Lisa Xu
    TI__Guru 53952 points

    Hi Lisa,

    Clemens is correct - the IBIS model for this device is most likely the best option we have for looking at the type of analysis that your customer is requesting.

    IBIS model is here: https://www.ti.com/lit/zip/sllm457 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Intellectual 505 points

    Hi Parker,Clemens:

    Thanks for your  the IBIS link share, I will feedback to you as soon as our SI team check.

    Best Regards 

    Lisa

  • 0 in reply to Lisa Xu
    TI__Guru 53952 points

    Hi Lisa,

    No problem - please let me know the feedback once you have it and if there are any other questions please let me know!

    Best,

    Parker Dodson