On 17/08/14 17:57, alb wrote:
> Hi Thomas,
>
> thomas.entner99@gmail.com wrote:
> []
>> To give some comments: Yes, we use 100 ohm termination, in fact on both
>> sides of the cable. I tried to remove the termination on both sides (but I
>> don't think I have removed both sides at the same time - would not really
>> make sense...). I have also checked with a multimeter and got the correct
>> values (e.g. 50 ohm, when both are mounted and cable connected.)
>
> I do not understand why you need termination at both ends. There are two
> aspects when terminating an LVDS line:
>
> 1. line impedence matching.
> 2. provide a load for a current source.
>
> In both cases it does not make too much sense to have a termination at the
> source.

See "LVDS Owner's Manual Design Guide , 4th Edition"
http://www.ti.com/lit/ml/snla187/snla187.pdf


> Did you see your signal on the scope *after* the receiver for both a /good/
> and /bad/ cable? Do you have an eye diagram?

Ensure the probes don't disturb the signal - which is easier said
than done.

Hi Thomas,

thomas.entner99@gmail.com wrote:
[]
> To give some comments: Yes, we use 100 ohm termination, in fact on both 
> sides of the cable. I tried to remove the termination on both sides (but I 
> don't think I have removed both sides at the same time - would not really 
> make sense...). I have also checked with a multimeter and got the correct 
> values (e.g. 50 ohm, when both are mounted and cable connected.)

I do not understand why you need termination at both ends. There are two 
aspects when terminating an LVDS line:

1. line impedence matching.
2. provide a load for a current source.

In both cases it does not make too much sense to have a termination at the 
source.

> My conclusion was, that with the original (quite strong) equalizer setting 
> things were barely working (even with "good" cables I got an transmission 
> error here and then while "bad" cables resulted in errors all over). I think 
> it was simply to strong for the short cable distance. With the lowest 
> possible equalizer setting we have not seen any transmission errors anymore 
> with any combination of boards and cables.

Did you see your signal on the scope *after* the receiver for both a /good/ 
and /bad/ cable? Do you have an eye diagram?

> 
> I had dismantled one cable and found that the (while they were really 
> twisted) the turn rate was very low, about 1 to 2 full turns about the 
> complete length. This might be good enough for long ethernet cables, but I 
> can imagine that this short length make some of them almost behave like 
> untwisted cables with quite some cross talk. 

twist per meter is usually specified in the cable datasheet; considering the 
rate you're working at I'd seriously consider the cable as a fundamental part 
to be analyzed.

> This is my explanation why 
> touching the untwisted wires had some influence although I am still 
> surprised that the effect was such a drastic improvement.

I would not call 'improvement' something that you cannot justify or explain. 
Since you do not know what is going on, it is *very* possible the problem is 
simply hidden for a various combination of causes and will likely reappear 
later on.

> What I still cannot understand is why the (in my opinion) much more dramatic 
> change of removing the termination resistors had no influence (not good nor 
> bad) on this obviously barely working system while the touching had.

IMO your connection is far from being a proper one, with serious matching 
issues and a very scattered amount of information to analyze. With these basis 
any conclusion on 'working' or 'not working' are pointless at best.

If you don't have a model to explain why it works and why it doesn't, your 
likely exposed to see the issue again later and thanks to Murphy's law it 
would be right two days before shipping!

If you want to seriously remove your issues you need to be able to explain 
every detail, spot the cause and remove it. Otherwise it would just be another 
piece of guesswork.

Al

Wow, quite some discussion going on here... (I have considered this myself =
"solved" and working on other stuff again.)

To give some comments:
Yes, we use 100 ohm termination, in fact on both sides of the cable. I trie=
d to remove the termination on both sides (but I don't think I have removed=
 both sides at the same time - would not really make sense...). I have also=
 checked with a multimeter and got the correct values (e.g. 50 ohm, when bo=
th are mounted and cable connected.)

My conclusion was, that with the original (quite strong) equalizer setting =
things were barely working (even with "good" cables I got an transmission e=
rror here and then while "bad" cables resulted in errors all over). I think=
 it was simply to strong for the short cable distance. With the lowest poss=
ible equalizer setting we have not seen any transmission errors anymore wit=
h any combination of boards and cables.

I had dismantled one cable and found that the (while they were really twist=
ed) the turn rate was very low, about 1 to 2 full turns about the complete =
length. This might be good enough for long ethernet cables, but I can imagi=
ne that this  short length make some of them almost behave like untwisted c=
ables with quite some cross talk. This is my explanation why touching the u=
ntwisted wires had some influence although I am still surprised that the ef=
fect was such a drastic improvement.

What I still cannot understand is why the (in my opinion) much more dramati=
c change of removing the termination resistors had no influence (not good n=
or bad) on this obviously barely working system while the touching had.

Regards,

Thomas

hi Jon,

Jon Elson <jmelson@wustl.edu> Wrote in message:
> Well, one other thing, is he terminating the LVDS with a resistor matching
> the characteristic impedance of the cable?  I'll bet he isn't, his results
> would VERY likely match what he describes.  We use 100 Ohm resistors, they
> are close enough.  For best results, tune between 110 and 120 Ohms for
> minimum reflection.

from the OP:

 I tried changing and even removing the termination resistors,
 this did not change the behavior at all. I have changed both
 boards, it is really the cable that makes the difference.

BTW, removing the terminators on an LVDS - single drop - line is
 *never* a good idea. LVDS drivers are current sources and the
 receiver may simply not tolerate the differential swing on it's
 big  input impedance.

I did not understand how the op solved his problem changing
 receiver equalization, he clearly mentioned some cables did work
 ok without the need to change equalization...

Al

Theo Markettos wrote:


> The OP stated he has one LVDS pair (D+ and D- twisted), and two UART
> signals
> (RXD and TXD).  The UART signals aren't sent by LVDS, they're just other
> things down the same cable.
OK, he confused  me with the UART data signals.

Well, one other thing, is he terminating the LVDS with a resistor matching
the characteristic impedance of the cable?  I'll bet he isn't, his results
would VERY likely match what he describes.  We use 100 Ohm resistors, they
are close enough.  For best results, tune between 110 and 120 Ohms for
minimum reflection.

Jon

Jon Elson <jmelson@wustl.edu> wrote:
> This is NOT LVDS!  The D stands for Differential!  You use one wire of the
> pair for the true signal, one wire for the complement signal.  I don't even
> know how you do single-ended with LVDS receivers, but it won't work well,
> as you clearly have discovered.
> 
> You can still use LVDS the right way with 3 pairs.  One pair is power &
> ground, one is the Rxd true/compl pair, the other is the Txd true/compl
> pair.

The OP stated he has one LVDS pair (D+ and D- twisted), and two UART signals
(RXD and TXD).  The UART signals aren't sent by LVDS, they're just other
things down the same cable.  I assume the OP's LVDS is unidirectional (or
else there would be other issues).  The OP's situation of having RXD+GND and
TXD+VCC pairs, rather than RXD+TXD and GND+VCC, is probably slightly better
but it's marginal at typical UART speeds (and as always depends on the setup
- as we've discovered the 'twisted pair' isn't).

You can connect single-ended into an LVDS receiver by grounding the negative
input (if you have positive and negative voltage rails) or putting it
mid-range via a resistive divider (on a single supply arrangement) but
you've just halved your signal amplitude (and thus differential SNR) - and
of course you have no protection from common-mode noise.

Theo

Den onsdag den 13. august 2014 23.02.24 UTC+2 skrev Jon Elson:
> thomas.entner99@gmail.com wrote:
> 
> 
> 
> 
> 
> > The cable is a "flat Ethernet cable" with 4 twisted pairs, one pair is
> 
> > unused, one pair is LVDS, one pair is GND + Rxd and the last is Power +
> 
> > TxD.
> 
> This is NOT LVDS!  The D stands for Differential!  You use one wire of the
> 
> pair for the true signal, one wire for the complement signal.  I don't even
> 
> know how you do single-ended with LVDS receivers, but it won't work well,
> 
> as you clearly have discovered.
> 
> 
> 
> You can still use LVDS the right way with 3 pairs.  One pair is power &
> 
> ground, one is the Rxd true/compl pair, the other is the Txd true/compl
> 
> pair.
> 

afaiu, he is using one pair for LDVS, the other rx/tx is single ended uart
paired with gnd and power. So the wiring is ok 

-Lasse

thomas.entner99@gmail.com wrote:


> The cable is a "flat Ethernet cable" with 4 twisted pairs, one pair is
> unused, one pair is LVDS, one pair is GND + Rxd and the last is Power +
> TxD.
This is NOT LVDS!  The D stands for Differential!  You use one wire of the
pair for the true signal, one wire for the complement signal.  I don't even
know how you do single-ended with LVDS receivers, but it won't work well,
as you clearly have discovered.

You can still use LVDS the right way with 3 pairs.  One pair is power &
ground, one is the Rxd true/compl pair, the other is the Txd true/compl
pair.

Jon

thomas.entner99@gmail.com wrote:
> I have an LVDS related issue that drives me crazy:
 
> There are two boards with a FPGA that are connected by a ca. 
> 30cm cable. Only 6 wirea are used:
> GND + Power
> LVDS (with embedded clock), 720Mbps
> UART (Rxd + Txd)
> (The cable is unshielded for flexibility reasons)
 
> The cable is a "flat Ethernet cable" with 4 twisted pairs, 
> one pair is unused, one pair is LVDS, one pair is GND + Rxd 
> and the last is Power + TxD.

Ethernet cables use pin pairs (1,2) (3,6) (4,5) and (7,8).

With the usual cables, you can see the pairs, including the 
colors, in the connector.

I have one, though, which I would call flat, and which the pairs
are not visible. It is very thin and flexible, and seems to work
fine for 100baseTX. I suspect the wire is much smaller than the
normal Cat 5 wire.

But otherwise, ethernet cables are normally rated to 100MHz.
1000baseT uses a complicated signal system on all four pairs
(in both directions at the same time) to get the bits through.

I wouldn't be surprised if you could get 720MHz through Cat 5
cable for 30cm, but your cable might not be quite that good.

-- glen

thomas.entner99@gmail.com wrote:
> It turned out that the equalization was too high. When increasing the
> setting, things got worse.  When setting it to "low", things work perfect
> without any errors...
> 
> I think the great effects of touching the cable can be explained with the
> weak twisting of the pairs.

I suspect the cable looks more like a parallel ribbon cable than twisted
pair.  That means there could be other signals between the twisted pair and
ground - eg if it turns out it goes:
GND RXD D+ D- TXD VCC

Any common mode currents, which there will be because your coding scheme is
not DC balanced, will flow around a loop with RXD or TXD in the middle, so
are likely to be inductively coupled with these wires (ie cause crosstalk). 
It's difficult to say what effect the inductance of this loop will have on
the LVDS signal (I suspect not a huge amount, but have no numbers: it will
depend on how quick the rise times are), but I wouldn't be surprised if you
saw coupling into the RXD and TXD wires.

Likewise, we'd have to know a bit more about the channel characteristics to
predict what the likely frequency response of the channel might be (you'd
model each part's LCR properties separately: hard numbers needed).

I'd guess that the touching effect increases both the capacitance to ground
and the inter-pair capacitance.  It moves the frequency nulls around - I'm
not familiar with the equalisation settings on that transceiver to know the
effect of the different modes, but it's not implausible that equalisation
could cope with one but not the other.  In theory your transmission line
should be correctly terminated so you don't need equalisation, but it very
obviously isn't.

Theo