Connect with us

Rework Schroff VME terminators to 3.3V

Discussion in 'Electronic Design' started by Joerg, Feb 24, 2007.

Scroll to continue with content
  1. Joerg

    Joerg Guest

    Hello Folks,

    We want to use a VME box that was meant for 5V VCC. The terminators are
    the electronic plug-in style gizmos from Schroff (60800 series). They
    have a L272M (power opamp) on there which per datasheet doesn't operate
    below 4V. We need to go to 3.3V. Has anyone found a quick and easy mod
    for this?
  2. John Larkin

    John Larkin Guest

    I'm not clear on what you want to do. Do you want to run the +5V power
    plane at 3.3? If so, what do you want the termination thevenin voltage
    to be?

    I assume this crate used a single resistor as the terminator at each
    end of the bus, driven by the opamp, as opposed to a classical
    power-hog thevenin termination (which was 330+470 ohms.)

  3. John B

    John B Guest

    I assume you're looking for 1.65V to drive the terminations. Couldn't
    you add a small -3.3V bias supply for the negative rail, as most of the
    current will be drawn from the +3.3V rail.
  4. Joerg

    Joerg Guest

    Yes, I have to run it at 3.3V because the other guys' logic stuff is
    operating there. About 60% or so would be good, to avoid being too close
    to the threshold. On my boards I'll use 74HC logic since we don't have
    to break any Mbit/sec records. Personally, I am a fan of efficiency and
    thus AC termination but I don't call the shots on this one. I found that
    most digital guys are scared of AC termination for some reason. Probably
    because of data-dependent jitter but if the C is small enough I never
    had any issues.

    Do you guys still run your VME stuff at 5V?

    BTW, we only use part of the bus because it's a serial comms scheme plus
    a dozen address lines.

    I think so but not sure since it is very densely packed and I wasn't
    able to bend a resistor array far enough. But on Monday I can ohm it out

    Guess I'll have to look for a L272 version or equivalent that can safely
    operate down to 3V.
  5. Joerg

    Joerg Guest

    More like 2V bias since you typically want to stay a bit off the
    thresholds in case someone lets a bus line go for too long or doesn't
    have Schmitts. There won't be any negative supply other than the usual
    (weak) -12V rail. I'll probably need a L272 pin-compatible power opamp
    that can live with 3V.
  6. Bob

    Bob Guest


    For SSTL and HSTL logic families, their termination supplies are commonly
    known as "VTT". They can and must be able to source as well as sink current.
    They are typically fixed at VDD/2 but some supplies allow other options.
    There are linear (opamps) and switching versions of these VTT supplies

    If you go to and search for "VTT" you'll see a bunch of these.
    Here is a linear version that I've used with good success:

  7. John  Larkin

    John Larkin Guest

    Heretic! VME is 5 volts! We use linear or switching regs on each
    module to get the 3.3, 2.5, and 1.2 volts (so far) that most of the
    logic needs. It's sort of nice to start at +5, because we can make the
    lower voltages very clean.

    One of our customers, name withheld, uses VME crates, name withheld,
    whose +5 spikes up to +9 if you tease the power switch just right. The
    old Xilinx 4000-series 5-volt FPGAs don't like that.
    What's the serial data rate? Keep in mind that most of the VME lines
    are actually under-terminated (typical loaded line Z < 100 ohms,
    equivalent termination 200 ohms at each end.)
    The standard VME thevenin termination is 200 ohms, one of same at each
    end of the bus.
    How about yanking the opamp and shorting the termination rail to 3.3?
    That would give you something like 200 ohms to +3.3 on each end of
    each bus line, perfectly usable.

    We've done dozens of various VME designs, using most every conceivable
    sync/async interface over lots of logic families, sold to scores of
    different customers, and VME always works. Pretty good for a
    25-year-old bus.

  8. Joerg

    Joerg Guest

    Full confession: In EE things I've been heretic before. A lot :)

    Yes but that would require level translators plus a switcher on the
    other folks' circuit boards. They'll pummel me in the next meeting even
    if I'd vow to design that for them. Well, maybe not if I bring a fresh
    bag of Noah's bagels.

    Reminds me of Jim's first switcher design where the boss did the "real"
    test, rocking the power switch up and down until a detonation was heard
    and molten solder splattered about.
    A few MHz, at the most. I like to terminate lines with their true
    impedance. Or do the crew cut thing with Schottky pairs but that's brute
    Should work. But maybe there is a pin-compatible power opamp that can
    take 3V VCC. Then I'd be home.

    It sure is a nice bus and IMHO it's here to stay. One reason being that
    systems designed for industrial use tend to remain in service for decades.
  9. Joerg

    Joerg Guest

    Thanks, I'll check that out.

    That one still requires 5V, like the L272 does. Which we don't have
    anymore. They even provided potmeters on those terminators so you can
    set your own VTT but unfortunately the L272 quits working correctly
    below 4V.
  10. John  Larkin

    John Larkin Guest

    Noah's things are fairly good, but they're not bagels, because they're
    not boiled. All that "New York" stuff is bogus.

    They do make a pretty good bagel dog. Every now and then a boy needs a
    bagel dog.

  11. Ben Jackson

    Ben Jackson Guest

    I don't know anything about VME (uh, don't leave gaps between cards?)
    but if you need to make 3.3V/2 you could look at any of the zillion VTT
    regulators made for SSTL applications. Many will work with inputs above
    2.5V and produce VDD/2. For example, LP2995.
  12. Joerg

    Joerg Guest

    Yeah, but I like 'em. The Shmear isn't my cup of tea although sometimes
    I use it.

    No idea what a bagel dog is but I guess it contains some kind of
    sausage. How do you make the sausage round?

    Every Sunday we have a meatless "bagel burger". A plain bagel cut in
    half and a slice of good cheese plus a fried egg in the middle. Don't
    tell the surgeon general...
  13. Joerg

    Joerg Guest

    Thanks, Ben. I could probably goose its VDDQ a bit to get it off the
    center. Generally it isn't good to center-terminate a bus because the
    millisecond someone lets a line go all receivers connected to it will
    chatter like crazy unless they are Schmitts.

    I'll keep looking for something in a DIP package though. I am not a fan
    of those hot-pad chips where it hisses when touching with a finger ;-)
  14. John  Larkin

    John Larkin Guest

    They bake a hot dog or a polish inside a bagel-bun sort of thing.
    Noah's are pretty good. In France they do a similar thing, a sausage
    baked inside a baguette.
    Sounds messy. Doesn't the egg yellow drool out of the bagel hole?

    We do lox (or the chunky native-Alaskan-style smoked salmon) and
    bagels once a week maybe. With cream cheese, sliced raw onions, and

    I figure there's just about enough omega 3's in the lox to null out
    the dairy fat in the cream cheese.

    Damn, I'm hungry.

  15. Joerg

    Joerg Guest

    We stir that in a bit and fry it just long enough so it solidifies,
    fried from both sides.

    Drool. Sounds delicious. My wife makes a German dish with lots of capers
    called "Koenigsberger Klopse". It was invented in Prussia, I believe the
    Koenigsberg area is now part of Lithuania (Kaliningrad). "Klops" would
    loosely translate to something like meat ball. I like them best with
    dumplings. Hand made, of course, non of this pre-cooked stuff.

    There is no English translation but this should explain it a bit:

    Non-German guests look at it with disgust at first but after a few bites
    they can't stop. However, the taste of the gravy is heavily on the sour
    side and needs to be acquired. Has to be. When I am done there is no
    gravy left. Not a speck.

    I am always hungry...
  16. joseph2k

    joseph2k Guest

    Sounds like millimodule time. Start with a dip header and build the circuit
    you need on top if it. With the right parts you should be able to get to
    1.75 V operation.
  17. Joerg

    Joerg Guest

    Yep. Digikey calls those "surfboards". Kind of cute, considering that
    they are not in California. The digital engineer really wants thevenin
    or at least hard resistive termination so I'll hafta do it. He granted
    my wish for a somewhat unorthodox bus scheme so it's give and take.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day