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Rework Schroff VME terminators to 3.3V

J

Joerg

Jan 1, 1970
0
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?
 
J

John Larkin

Jan 1, 1970
0
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?

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
 
J

John B

Jan 1, 1970
0
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?

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.
 
J

Joerg

Jan 1, 1970
0
John said:
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?

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 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.)

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.
 
J

Joerg

Jan 1, 1970
0
John said:
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.

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.
 
B

Bob

Jan 1, 1970
0
Joerg said:
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.

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

http://focus.ti.com/lit/ds/symlink/tps51100.pdf


Bob
 
J

John Larkin

Jan 1, 1970
0
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?

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

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.)
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.

The standard VME thevenin termination is 200 ohms, one of same at each
end of the bus.
Guess I'll have to look for a L272 version or equivalent that can safely
operate down to 3V.

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.

http://en.wikipedia.org/wiki/VMEbus


John
 
J

Joerg

Jan 1, 1970
0
John said:
Heretic! VME is 5 volts! ...


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

... 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.

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.

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.

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.
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.)

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.
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.

Should work. But maybe there is a pin-compatible power opamp that can
take 3V VCC. Then I'd be home.

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.

http://en.wikipedia.org/wiki/VMEbus

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.
 
J

Joerg

Jan 1, 1970
0
Bob said:
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 www.ti.com and search for "VTT" you'll see a bunch of these.
Here is a linear version that I've used with good success:

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.
 
J

John Larkin

Jan 1, 1970
0
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.

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
 
B

Ben Jackson

Jan 1, 1970
0
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). [...]
We need to go to 3.3V.

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.
 
J

Joerg

Jan 1, 1970
0
John said:
Noah's things are fairly good, but they're not bagels, because they're
not boiled. All that "New York" stuff is bogus.

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

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

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...
 
J

Joerg

Jan 1, 1970
0
Ben said:
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). [...]
We need to go to 3.3V.


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.

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 ;-)
 
J

John Larkin

Jan 1, 1970
0
No idea what a bagel dog is but I guess it contains some kind of
sausage. How do you make the sausage round?

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.
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...

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
 
J

Joerg

Jan 1, 1970
0
John said:
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 stir that in a bit and fry it just long enough so it solidifies,
fried from both sides.

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.

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:
http://www.myhouseandgarden.com/recipes/koenigsberger_klopse.htm

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 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.

I am always hungry...
 
J

joseph2k

Jan 1, 1970
0
Joerg said:
Ben said:
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). [...]
We need to go to 3.3V.


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.

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 ;-)

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.
 
J

Joerg

Jan 1, 1970
0
joseph2k said:
Joerg wrote:

Ben Jackson wrote:

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). [...]
We need to go to 3.3V.


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.

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 ;-)


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.

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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