Rework Schroff VME terminators to 3.3V

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.)

John

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.

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John B

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 somehow.

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

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Regards, Joerg

http://www.analogconsultants.com

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.

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Regards, Joerg

http://www.analogconsultants.com

Joerg,

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 available.

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:

Bob

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.

John

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 force.

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.

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Regards, Joerg

http://www.analogconsultants.com

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.

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Regards, Joerg

http://www.analogconsultants.com

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.

John

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.

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Ben Jackson AD7GD

http://www.ben.com/

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...

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Regards, Joerg

http://www.analogconsultants.com

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 ;-)

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Regards, Joerg

http://www.analogconsultants.com

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 capers.

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.

John

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...

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Regards, Joerg

http://www.analogconsultants.com

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.

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 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

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.

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Regards, Joerg

http://www.analogconsultants.com