"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
Bidirectional LVDS
Started by ●June 27, 2007
Reply by ●June 27, 20072007-06-27
Reply by ●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 ●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 ●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 ●June 30, 20072007-06-30
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. > >AndyMost 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
