Bidirectional LVDS

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

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

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

I was contemplating the former as shown in National's application (Fig

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).

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

YMMV ;)

Steve

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

In comp.arch.fpga Steve at fivetrees wrote: ...

That's what Bus-LVDS for.

Called also M-LVDS, LVDM or ...

Cheers! Rich

Oooh. *Great* timing. I was just looking for some twisted-pair ribbon cable blurb... ta!

Steve

Thanks! :-)

Sometimes I'm kinda psycho^Hic. ;-)

Cheers! Rich

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.

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