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?
Bidirectional LVDS
Started by ●June 27, 2007
Reply by ●June 27, 20072007-06-27
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
Reply by ●June 27, 20072007-06-27
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. > > AndyThe second device draws its power from the first, so they share a common ground.
Reply by ●June 27, 20072007-06-27
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. :-) Good Luck! Rich
Reply by ●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. > > 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. Steve http://www.fivetrees.com
Reply by ●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).
Reply by ●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 ●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 ●June 27, 20072007-06-27
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 ●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 ;)http://www.belden.com/pdfs/03Belden_Master_Catalog/07Flat_Cable/07FlatCable.pdf page 9. :-) Cheers! Rich
