On Wed, 27 Jun 2007 09:09:45 -0700, Andy <jonesandy@comcast.net>
wrote:
>On Jun 27, 10:39 am, Richard Henry <pomer...@hotmail.com> wrote:
>> I need to extend a memory-mapped bus into another enclosure and
>> thought that a bidirectional LVDS implementation with serial/
>> deserializer pairs at each end might work. Does anyone have any
>> experience or guidance on such a setup?
>
>LVDS has a really low common mode voltage tolerance, so if you use it
>between enclosures, make very sure you have an excellent grounding
>scheme and control of return currents between enclosures. Generally
>not a good application of LVDS without some means of improving CMV
>tolerance.
>
>Andy
Most of the LVDS receivers that we've tested, including the Xilinx
Spartan3 parts, seem to behave like excellent, very fast rail-to-rail
comparators, at a fraction of the price of an officialy-designated
comparator of similar performance. They do generally have a deliberate
input offset, in the 10's of millivolts sort of range. Most of them
make good zero-crossing detectors, signal on one input pin and ground
on the other.
Still, one wouldn't want to drive any input hard much beyond its local
rails.
Return currents?
John
Reply by Uwe Bonnes●June 30, 20072007-06-30
In comp.arch.fpga Richard Henry <pomerado@hotmail.com> wrote:
> On Jun 27, 1:06 pm, Uwe Bonnes <b...@hertz.ikp.physik.tu-darmstadt.de>
> wrote:
> > In comp.arch.fpga Steve at fivetrees <s...@nospamtafivetrees.com> wrote:
> > ...
> >
> > > To the OP re bidirectional: it's not clear to me whether you expect to
> > > run two loops (one outgoing, one incoming), or shove data down one pair
> > > of wires in boith directions. If the latter, I'm not clear on where
> > > you'd put the receiver load - at both ends? If so, expect to see half
> > > the voltage across each receiver.
> >
> > That's what Bus-LVDS for.
> >
> > Called also M-LVDS, LVDM or ...
> How about a hybrid solution? I don't want to run so many wires (I
> don't have room for such a big connector). Could I gang up 4 signals
> per pair without a lot of overhead, and still keep it bidirectional?
You can use any protocoll and multiplex the lines as you like. But that will
need protocoll overhead.
Look e.g. at TI or National, what can be done with LVDS.
Bus LVDS is anyways a good choice, as is has stronger send levels and more
sensitive receive levels.
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
Reply by Richard Henry●June 29, 20072007-06-29
On Jun 27, 1:06 pm, Uwe Bonnes <b...@hertz.ikp.physik.tu-darmstadt.de>
wrote:
> In comp.arch.fpga Steve at fivetrees <s...@nospamtafivetrees.com> wrote:
> ...
>
> > To the OP re bidirectional: it's not clear to me whether you expect to
> > run two loops (one outgoing, one incoming), or shove data down one pair
> > of wires in boith directions. If the latter, I'm not clear on where
> > you'd put the receiver load - at both ends? If so, expect to see half
> > the voltage across each receiver.
>
> That's what Bus-LVDS for.
>
> Called also M-LVDS, LVDM or ...
How about a hybrid solution? I don't want to run so many wires (I
don't have room for such a big connector). Could I gang up 4 signals
per pair without a lot of overhead, and still keep it bidirectional?
Reply by Rich Grise●June 27, 20072007-06-27
On Thu, 28 Jun 2007 01:22:21 +0100, Steve at fivetrees wrote:
> "Rich Grise" <rich@example.net> wrote in message
> news:pan.2007.06.27.21.55.47.193677@example.net...
>> On Wed, 27 Jun 2007 20:51:42 +0100, Steve at fivetrees wrote:
>>> "Rich Grise" <rich@example.net> wrote in message
>> ...
>>>> use ordinary ribbon cable but with twice as many leads as signals,
>>>> and
>>>> make every other line ground.
>>>>
>>>> I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
>>>> low bus speeds.
>>>
>>> Further to earlier response: re-reading this carefully, I'd agree if
>>> we
>>> were talking about normal (TTL-esque) digital signals. But with LVDS,
>>> it
>>> *is* a differential current loop. A twisted pair is ideal, otherwise
>>> I'd
>>> put the two conductors side by side in a ribbon with a guard ground
>>> either side.
>>>
>>> YMMV ;)
>>
>> http://www.belden.com/pdfs/03Belden_Master_Catalog/07Flat_Cable/07FlatCable.pdf
>> page 9. :-)
>
> Oooh. *Great* timing. I was just looking for some twisted-pair ribbon
> cable blurb... ta!
Thanks! :-)
Sometimes I'm kinda psycho^Hic. ;-)
Cheers!
Rich
Reply by Steve at fivetrees●June 27, 20072007-06-27
"Rich Grise" <rich@example.net> wrote in message
news:pan.2007.06.27.21.55.47.193677@example.net...
> On Wed, 27 Jun 2007 20:51:42 +0100, Steve at fivetrees wrote:
>> "Rich Grise" <rich@example.net> wrote in message
> ...
>>> use ordinary ribbon cable but with twice as many leads as signals,
>>> and
>>> make every other line ground.
>>>
>>> I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
>>> low bus speeds.
>>
>> Further to earlier response: re-reading this carefully, I'd agree if
>> we
>> were talking about normal (TTL-esque) digital signals. But with LVDS,
>> it
>> *is* a differential current loop. A twisted pair is ideal, otherwise
>> I'd
>> put the two conductors side by side in a ribbon with a guard ground
>> either side.
>>
>> YMMV ;)
>
> http://www.belden.com/pdfs/03Belden_Master_Catalog/07Flat_Cable/07FlatCable.pdf
> page 9. :-)
Oooh. *Great* timing. I was just looking for some twisted-pair ribbon
cable blurb... ta!
Steve
http://www.fivetrees.com
Reply by Rich Grise●June 27, 20072007-06-27
On Wed, 27 Jun 2007 20:51:42 +0100, Steve at fivetrees wrote:
> "Rich Grise" <rich@example.net> wrote in message
...
>> use ordinary ribbon cable but with twice as many leads as signals, and
>> make every other line ground.
>>
>> I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
>> low bus speeds.
>
> Further to earlier response: re-reading this carefully, I'd agree if we
> were talking about normal (TTL-esque) digital signals. But with LVDS, it
> *is* a differential current loop. A twisted pair is ideal, otherwise I'd
> put the two conductors side by side in a ribbon with a guard ground
> either side.
>
> YMMV ;)
In comp.arch.fpga Steve at fivetrees <steve@nospamtafivetrees.com> wrote:
...
> To the OP re bidirectional: it's not clear to me whether you expect to
> run two loops (one outgoing, one incoming), or shove data down one pair
> of wires in boith directions. If the latter, I'm not clear on where
> you'd put the receiver load - at both ends? If so, expect to see half
> the voltage across each receiver.
That's what Bus-LVDS for.
Called also M-LVDS, LVDM or ...
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
Reply by Gabor●June 27, 20072007-06-27
On Jun 27, 3:43 pm, Richard Henry <pomer...@hotmail.com> wrote:
> On Jun 27, 12:35 pm, "Steve at fivetrees" <s...@NOSPAMTAfivetrees.com>
> wrote:
>
>
>
> > "Rich Grise" <r...@example.net> wrote in message
>
> >news:pan.2007.06.27.18.27.35.295523@example.net...
>
> > > On Wed, 27 Jun 2007 10:40:01 -0700, Richard Henry wrote:
> > >> On Jun 27, 9:09 am, Andy <jonesa...@comcast.net> wrote:
> > >>> On Jun 27, 10:39 am, Richard Henry <pomer...@hotmail.com> wrote:
>
> > >>> > I need to extend a memory-mapped bus into another enclosure and
> > >>> > thought that a bidirectional LVDS implementation with serial/
> > >>> > deserializer pairs at each end might work. Does anyone have any
> > >>> > experience or guidance on such a setup?
>
> > >>> LVDS has a really low common mode voltage tolerance, so if you use
> > >>> it
> > >>> between enclosures, make very sure you have an excellent grounding
> > >>> scheme and control of return currents between enclosures. Generally
> > >>> not a good application of LVDS without some means of improving CMV
> > >>> tolerance.
>
> > >> The second device draws its power from the first, so they share a
> > >> common ground.
>
> > > How far? With proper terminations, depending on the bus speed, you
> > > could
> > > use ordinary ribbon cable but with twice as many leads as signals, and
> > > make every other line ground.
>
> > > I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
> > > low bus speeds.
>
> > > YMMV. :-)
>
> > Erm... one (single-ended) application of LVDS I know of shoves a 25MHz
> > clock down some ribbon cable... 10m of it. Seems to work. LVDS, being a
> > current loop, tolerates this kind of (ab)use better than most.
>
> > To the OP re bidirectional: it's not clear to me whether you expect to
> > run two loops (one outgoing, one incoming), or shove data down one pair
> > of wires in boith directions. If the latter, I'm not clear on where
> > you'd put the receiver load - at both ends? If so, expect to see half
> > the voltage across each receiver.
>
> > Stevehttp://www.fivetrees.com-Hide quoted text -
>
> > - Show quoted text -
>
> I was contemplating the former as shown in National's application (Fig
> 2.4):
>
> http://www.national.com/appinfo/lvds/files/lvds_ch2.pdf
>
> It also has the precaution about lower voltages, but it predicts a 10m
> limit (my app is much shorter) and cautions against a noisy
> environment (one enclosure includes an rf transmitter).
If you're going to use SERDES on the LVDS lines, I would assume your
bit rate is fairly high. In this case I would advise against the
bidirectional wire idea. Save yourself the headaches and use one
set of wires for each direction. Certainly it is possible to use
the circuit of figure 2.4, but you won't be able to change direction
quickly, in addition to the reduced signal strength mentioned. The
point of SERDES is to reduce the number of pairs significantly, so
it shouldn't cost too much to assign the additional pairs to run
independent one-way data paths.
Just my 2 cents,
Gabor
Reply by Steve at fivetrees●June 27, 20072007-06-27
"Rich Grise" <rich@example.net> wrote in message
news:pan.2007.06.27.18.27.35.295523@example.net...
> On Wed, 27 Jun 2007 10:40:01 -0700, Richard Henry wrote:
>> On Jun 27, 9:09 am, Andy <jonesa...@comcast.net> wrote:
>>> On Jun 27, 10:39 am, Richard Henry <pomer...@hotmail.com> wrote:
>>>
>>> > I need to extend a memory-mapped bus into another enclosure and
>>> > thought that a bidirectional LVDS implementation with serial/
>>> > deserializer pairs at each end might work. Does anyone have any
>>> > experience or guidance on such a setup?
>>>
>>> LVDS has a really low common mode voltage tolerance, so if you use
>>> it
>>> between enclosures, make very sure you have an excellent grounding
>>> scheme and control of return currents between enclosures. Generally
>>> not a good application of LVDS without some means of improving CMV
>>> tolerance.
>>
>> The second device draws its power from the first, so they share a
>> common ground.
>
> How far? With proper terminations, depending on the bus speed, you
> could
> use ordinary ribbon cable but with twice as many leads as signals, and
> make every other line ground.
>
> I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
> low bus speeds.
Further to earlier response: re-reading this carefully, I'd agree if we
were talking about normal (TTL-esque) digital signals. But with LVDS, it
*is* a differential current loop. A twisted pair is ideal, otherwise I'd
put the two conductors side by side in a ribbon with a guard ground
either side.
YMMV ;)
Steve
http://www.fivetrees.com
Reply by Richard Henry●June 27, 20072007-06-27
On Jun 27, 12:35 pm, "Steve at fivetrees" <s...@NOSPAMTAfivetrees.com>
wrote:
> "Rich Grise" <r...@example.net> wrote in message
>
> news:pan.2007.06.27.18.27.35.295523@example.net...
>
>
>
>
>
> > On Wed, 27 Jun 2007 10:40:01 -0700, Richard Henry wrote:
> >> On Jun 27, 9:09 am, Andy <jonesa...@comcast.net> wrote:
> >>> On Jun 27, 10:39 am, Richard Henry <pomer...@hotmail.com> wrote:
>
> >>> > I need to extend a memory-mapped bus into another enclosure and
> >>> > thought that a bidirectional LVDS implementation with serial/
> >>> > deserializer pairs at each end might work. Does anyone have any
> >>> > experience or guidance on such a setup?
>
> >>> LVDS has a really low common mode voltage tolerance, so if you use
> >>> it
> >>> between enclosures, make very sure you have an excellent grounding
> >>> scheme and control of return currents between enclosures. Generally
> >>> not a good application of LVDS without some means of improving CMV
> >>> tolerance.
>
> >> The second device draws its power from the first, so they share a
> >> common ground.
>
> > How far? With proper terminations, depending on the bus speed, you
> > could
> > use ordinary ribbon cable but with twice as many leads as signals, and
> > make every other line ground.
>
> > I've made this work with 3 feet (~1m) of ribbon cable, but at fairly
> > low bus speeds.
>
> > YMMV. :-)
>
> Erm... one (single-ended) application of LVDS I know of shoves a 25MHz
> clock down some ribbon cable... 10m of it. Seems to work. LVDS, being a
> current loop, tolerates this kind of (ab)use better than most.
>
> To the OP re bidirectional: it's not clear to me whether you expect to
> run two loops (one outgoing, one incoming), or shove data down one pair
> of wires in boith directions. If the latter, I'm not clear on where
> you'd put the receiver load - at both ends? If so, expect to see half
> the voltage across each receiver.
>
> Stevehttp://www.fivetrees.com- Hide quoted text -
>
> - Show quoted text -
I was contemplating the former as shown in National's application (Fig
2.4):
http://www.national.com/appinfo/lvds/files/lvds_ch2.pdf
It also has the precaution about lower voltages, but it predicts a 10m
limit (my app is much shorter) and cautions against a noisy
environment (one enclosure includes an rf transmitter).