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Differential Pair Setup of Class to Class

Sagetech

Trying to configure differential pair spacing from pair to pair and other traces.

I've been following the Parallel Systems Differential Pair tutorial but I'm not getting the results I need. 

I have my diff pair trace width and spacing set up in the physical cset.

I set up a spacing cset and net class to get my pair to pair spacing constrained. When I do this, the pairs do seem to adhere to the pair to pair spacing constraint but I get a DRC on the differential pair line to line spacing set in the physical constraint. If I remove the spacing cset, the pair spacing DRC goes away but the pair to pair spacing is no longer constrained and I can route one pair too close to the next pair.

Been through the instructions a dozen times with the same result. Instructions were published in 2020, maybe things have changed since then?

Thoughts / suggestions?

Tom

  • avant
    0 in reply to Sagetech

    Try assigning each pair (or net group) a unique net class name.

    Then go to the CSet assignment matrix and assign a spacing rule at the intersections of the row and colums of the net classes you want to control.

  • Sagetech
    0 in reply to avant

    So if I have 50 differential pairs I need to assign a different "class" name to each - seems time consuming..........

    I'm sure I don't understand all of the groups and class assignments and method to define. I've watched many videos and read many instructions but keep getting errors

    I simply want to have the trace to trace spacing within my differential pair to be 6 mil and the spacing from differental pair to any other trace or differentail pair to be 18 mil.

    I can get the diff pair spacing set correctly, but when I apply the 18 mil spacing using the class to class I get the correct 18 mil spacing when routing but I then get a DRC for the 6 mil trace to trace spacing within the diff pair because the DRC wants it to be 18 mil now.

    So confusing............

  • avant
    0 in reply to Sagetech

    I tried going through the convoluted, confusing method as well, and gave up. It seems like it should be a simple task, and diff-pair constraints would take precedent over class to class rules.

    My method is easy, although time consuming. 

    I hope someone here has a better solution. 

  • Sagetech
    0 in reply to avant

    Thanks for the reply and letting me know I'm not going crazy.

    I've been using Orcad since it ran on floppy disks (back in the 80's) and sometimes can still make me feel very stupid.........

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