[SI-LIST] AW: Wiring Harnessing SI Question

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: "movax@xxxxxxxxx" <movax@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 26 Nov 2015 09:01:18 +0000

Don't think too much about Return currents, your Problem is signal reference.
When conductors are floating free, then especially the clock signals will
couple into every conductor that is close enough to couple (Clock has the
highest frequency content especially with fast rising edges). Depending on
cable length and relative position clock will strongly couple into other lines
(Data or different Clocks). You should be able to see those using an simple
scope with high impedance Probes. The easiest way to improve your system is to
use either coaxial or paired Cable including a solid reference (GND) or to use
much thicker cable isolation (or different wire harnesses) to add distance
between the signal wire and other wires. In a multi wire Cable, you can
separate these wires by assigning wires optimally, but for free wire harness
that’s a problem.

BR
Gert



Besuchen Sie uns auf der SPS/IPC/DRIVES 2015 vom 24. bis 26. November 2015 in
Halle 10, Stand 140.
Entdecken Sie unsere Ideen und Lösungen zu Integrated Industry - „HAII4YOU –
HARTING Integrated Industry 4 You“. http://www.HARTING.de/SPS


----------------------------------------
Absender ist HARTING KGaA; Marienwerderstraße 3, D-32339 Espelkamp;
Registergericht: Bad Oeynhausen; Register-Nr.: HRB 8809; Vorsitzender des
Aufsichtsrats: Dipl.-Kfm. Jörg Selchow; persönlich haftende Gesellschafter:
Dipl.-Kfm. Dr.-Ing. E.h. Dietmar Harting, Philip F.W. Harting, Maresa
Harting-Hertz; HARTING WiMa AG (Luxemburg) & Co. KG, HARTING Beteiligungs GmbH
& Co. KG; Generalbevollmächtigte Gesellschafterin: Dipl.-Hdl. Margrit Harting

-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Krunal Desai
Gesendet: Mittwoch, 25. November 2015 19:29
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Wiring Harnessing SI Question

HI all --

Hoping that list can shed some light on some wire harness related SI questions
at some very, very low speeds.

This list has done a good job teaching me about SI in general, and especially
the concept that while DC (i.e. power delivery) enjoys the path of least
resistance, the same is not true for AC signals (I think rule of thumb says
above 100kHz?).

We have a design where we have a 50-pin connector to our DUT on one end. This
goes to a Y-adapter which turns into 2x 32-pin MIL-STD circular connectors,
then a 6-8ft harness, and then our host / test equipment box. Earlier this
year, we decided to make one cable power, and one cable data -- logical, right?

Recently though, we've observed severe SI issues when running two data
interfaces simultaneously (1MHz JTAG, and I2C) and this got me thinking, and
re-reading what we designed. Turns out that we were very thorough in our power
and data separation -- there are no grounds in the data harness at all! This
reminded me again of AC return currents and their wanting to minimize loop area.

Here is my theory / gut feeling -- as the host box clocks out an edge on lets
say TCK, which is driven by a fairly "slow" driver (~15ns rise time I think),
at a frequency of 1MHz, it will leave the driver, travel on a PCB trace, hop
onto the harness, traverse the harness, hop onto the DUT PCB and into the input
device. Not to anthropomorphize current too much, but at this point having made
the long trip to the DUT, he really wants to get home as fast as possible (and
in as small a loop area as possible). Unfortunately for him, the DC ground
return path is "far" away from the path he arrived on (the two cables kind of
flop around near each other), so he's going to take some path back in the
signal harness -- which could unfortunately be some output driver that happens
to be at '0', or some other data line. This isn't a ribbon harness, so the
conductors just float around in space constrained only by braid. I think on my
scope I can see odd / increased noise when certain lines are low -- does that
low impedance path to GND encourage return current to flow on that line? If I
idle all I2C traffic, I get no errors on JTAG whatsoever, but when I2C traffic
begins to occur, my JTAG transaction quickly become corrupted.
This is the majority case; I've also observed the inverse behavior where JTAG
squashes I2C.

Am I on the right track here? Or should I be looking more for reflections /
impedance mismatches? This is not a fast interface overall -- the edge rates
aren't ultra-fast and I2C is an open-drain, slow bus! (I have heard stories of
people tuning the VOL strength of their I2C devices before).

Additionally, I swear I remember reading somewhere an article (or maybe it was
on the list) for a good rule of thumb on how many return conductors to provide
for a given signal harness with n signal conductors -- does anyone recall this?

Thanks! I'm hoping to work out / test this problem over Thanksgiving
-- it's a frustrating one, but those are also the ones that give you the best
feeling when you defeat them.

KD
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:
//www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list

List archives are viewable at:
//www.freelists.org/archives/si-list

Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu


Other related posts: