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Standard backplanes with full ground plane?

Discussion in 'Electronic Design' started by Joerg, Feb 12, 2007.

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  1. Joerg

    Joerg Guest

    For prototyping a large embedded system I am looking for a card rack
    with a good backplane. 20 or more slots, 64 or 96 pin DIN connector or
    similar, single height, maybe Eurocard-size. Preferably not Press-Fit
    but we can always hand solder, of course. Most of what's available is
    VME with super-wide power and ground traces but as far as I could make
    out from the data sheets no full ground plane. We've got analog stuff on
    most of the cards so a ground plane is important. The bottom
    (solder-side) layer should be traces so we can cut some for slot ID and
    analog stuff, can't use 100% internally bussed versions.

    Short of laying out our own (again...), is there something with a ground
    plane that can be had off-the-shelf?
  2. Del Cecchi

    Del Cecchi Guest

    People actually still make backplanes without ground planes in them? You
    sure about that?

    Try buying maybe one of the backplanes made for blade servers. Gotta be
    generic stuff out there.

    You can't even do controlled impedence traces without planes.

  3. Joerg

    Joerg Guest

    Blade servers, good point. I'll look if they have some that are 20-wide
    or more.
  4. John  Larkin

    John Larkin Guest

    VME backplanes have ground planes, and we do all sorts of analog stuff
    in VME. Never had a problem we could blame on a backplane, even using
    a motley collection of ebay crates and a bazillion different customer

    The slot ID thing would be a challenge with 3U VME. No problem in 6U.

  5. Actually you can. "Coplanar transmission line" is the word.
    A 5mil gap to either side and you get an impedance.

  6. Joerg,
    I suggest making your own. Standard backplanes have
    standard distances. Even the worst autorouter can't do
    anything wrong.

  7. Joerg

    Joerg Guest

    Yes, meantime I found some with planes. But all traces inside, can't
    unhook some of the DIN pins without major surgery. I guess we'll roll
    our own again then.

    Nice! Interesting that you guys tend not to use the 2nd rail.

    6U is too monstrous for this case. It's not just ID we need but also
    some near-realtime signaling from the boards to a host where the usual
    IRQ1 arbitration would take too long.

    Question: Since you don't seem to use slot ID how do identical boards in
    one rack know which position they are in? DIP switches? Or maybe they
    don't have to know in your cases?
  8. Joerg

    Joerg Guest

    We probably will. There comes a point where it's more economical to just
    whip up a board rather than searching for hours. I just hope that my
    layouter has a VME template with all the connector positions and holes
    on his CAD. Then it would be really easy. Another thing I had done
    before is just solder one by hand. No big deal if the iron has 100W+ but
    the stench from that activity was quite nasty.
  9. Del Cecchi

    Del Cecchi Guest

    not very controlled unless you have control of above and below. And via
  10. Joerg

    Joerg Guest

    During my search I came across quite a few of these. Mostly called
    "Faraday shielded", guess because the marketeers thought that sounds
    more fancy. They also staggered the lines so they are offset to each
    other between planes. Well, to some extent. It might be a bit of a
    white-knuckle ride on a 21-slot board.
  11. John Larkin

    John Larkin Guest

    I wonder how many backplanes have been invented over the years. I've
    done a few myself.

    The analog stuff is mostly slow, so we don't really need 32 bit
    transfers. Leaves more room on the board for stuff!
    Doesn't CPCI have geographic addressing? Those backplanes are mass
    produced, too.
    Dip switches. Nobody seems to mind.

  12. John Larkin

    John Larkin Guest

    We have the basic VME board layouts, in PADS, if that would help.

  13. Joerg

    Joerg Guest

    In ultrasound we ended up making our own. Every single time. Lack of
    slot ID was just one reason, there just weren't enough configurable
    lines, or in cases like VME there were none at all.
    But they are overkill with their 220 pin connectors and you can usually
    only get them with 8 slots. Plus they are, gasp, metric ;-)
    In this case that is bound to cause trouble because there will be dozens
    of boards that are going to be swapped a lot. DIP switches are also not
    reliable IMHO. When using the ADN8831 eval board for a TEC controller I
    was almost done tuning the PID when one broke off. Luckily someone had a
    sewing needle so I could move that position one more time.
  14. Joerg

    Joerg Guest

    Thanks, it might help since my layouter also uses PADS. He just told me
    that he has done customized VME-type backplanes and has the important
    outlines etc. but that the dimensions are different between rack
    manufacturers. Meaning one VME backplane may not fit into the rack of
    another vendor. Oh man, did they screw up that standard as well?
  15. joseph2k

    joseph2k Guest

    The more i read, the more it seems, that what you want is VXI bus instead.
  16. Joerg

    Joerg Guest

    Well, for the protoype my client got a hold of old-style VME via EBay.
    After that we'll lay out our own anyway because a typical bus backplanes
    is prohibitively expensive. The old ones have bottom and top traces so
    we can cut and wire for a prototype system. So we'll have automatic slot
    detection from day one :)
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