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Bidirectional LVDS

R

Richard Henry

Jan 1, 1970
0
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?
 
A

Andy

Jan 1, 1970
0
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
 
R

Richard Henry

Jan 1, 1970
0
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.
 
R

Rich Grise

Jan 1, 1970
0
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
 
S

Steve at fivetrees

Jan 1, 1970
0
Rich Grise said:
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
 
R

Richard Henry

Jan 1, 1970
0
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).
 
S

Steve at fivetrees

Jan 1, 1970
0
Rich Grise said:
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
 
G

Gabor

Jan 1, 1970
0
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
 
U

Uwe Bonnes

Jan 1, 1970
0
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 ...
 
R

Rich Grise

Jan 1, 1970
0
Oooh. *Great* timing. I was just looking for some twisted-pair ribbon
cable blurb... ta!

Thanks! :)

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

Cheers!
Rich
 
R

Richard Henry

Jan 1, 1970
0
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?
 
U

Uwe Bonnes

Jan 1, 1970
0
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.
 
J

John Larkin

Jan 1, 1970
0
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
 
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