[SI-LIST] Re: Wiring Harnessing SI Question
- From: Krunal Desai <movax@xxxxxxxxx>
- To: leeritchey@xxxxxxxxxxxxx
- Date: Wed, 25 Nov 2015 13:24:24 -0800
Thanks for the replies all --
On Wed, Nov 25, 2015 at 12:05 PM, Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote:
Remember that "ground" doesn't have any meaning in the context of harnesses
like this. Transmission lines need partners such as shields on which their
return currents can flow. These shields also prevent cross talk. Probably
what will fix this problem.
This may be a more philosophical question -- do I treat these as
transmission lines despite the relatively 'slow' edge rates / clock
speeds I'm working with? I have no personal objection to this, but my
knowledge-base is such that I have a hard time explaining to people
why I'm treating a bus such as I2C (which is generally an
afterthought) as a transmission line. Certainly on a PCB, I don't
think much about it outside of routing it cleanly / minimizing plane
splits / jumps / etc.
Re: shielding, I should offer more detail on the harness -- each has a
few twisted shielded pairs in there for Ethernet and RS-422, and these
interfaces are operating fine. The rest are all single-ended 26AWG
conventional PVC-insulated wire.
On Wed, Nov 25, 2015 at 11:01 AM, Chuck Corley <corley@xxxxxxxxx> wrote:
It doesn't sound like the impedance of the traces and cables is being
controlled? Or signal termination is being used? That typically leads to
signal reflections, ringing, etc.
For JTAG, I have series termination at the drivers (3 at the test
equipment, 1 at the DUT for TDO) that tries to match to 50 ohm. I2C I
don't have any -- of course the bus has pull-up resistors on it, but
that is about it. I note that at the test equipment, TCK has a nasty
knee / signs of reflection, but at the DUT, the edge looks quite
clean.
Your outgoing signals are being split in the Y connector? That changes
whatever the impedance was at that point, possibly causing reflections, etc.
It's a single connector on one end, and then it turns into two
circulars -- all the data signals are heading to the same circular
connector.
But the main problem is probably crosstalk. Your thoughts about the return
currents are on the right track, but that's part of the problem, overall it
sounds like you're probably having crosstalk problems in the traces and
cables. If there is no adjacent reference return signal then there is
nothing to set the cable/trace impedance and act as a current return path, so
your active signal edges will crosstalk their way into adjacent signals like
what you say you're seeing. Maybe do a test to check which signals are
adjacent in the cable/board to see if they match the signals being corrupted,
and you'll probably confirm it's an adjacent signal crosstalk problem.
I've confirmed some aggressor / victim pairs, but additionally I've
confirmed that on piece of equipment, there's no crosstalk being
induced by the layouts. Firing up 1MHz TCK superimposes 1MHz noise on
SCL while active which is easily detectable -- it doesn't seem to be
the magnitude to cause problems, but it is definitely there.
The lack of reference return paths ("grounds") in the data harness sounds
like a problem. The number of grounds is situation/application dependent.
But for one example, on some slow applications like yours where non-shielded
cables are used I've sometimes had to alternate every other line as
ground/signal/ground/signal, etc. in order to set some kind of a known signal
impedance and current return path for the signals traveling in the cable.
This sounds more like what your data cable might have needed.
It's late in the project (naturally), but I wonder if a hot-fix I
could do would be trying to twist each of my AC signals with a GND
wire in the harness length, and then tie all those GNDs together at
either end (to avoid having to add 6-8 pins from thin air).
In the example you're giving, was that a ribbon cable application? Or
did you actually twist certain cables together to ensure they
presented a known impedance (I assume that would be determined by the
turn ratio).
Thanks for all the help!
KD
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