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For my project I currently use 2x8 IDC cables, all of them carrying digital data, like SPI, I2C and TDM (digital audio). As I have tested, I found that these cables bring in loads of noise to the system and disturb the signal integrity, and I do not get the signals at the other end of the cable.

Note: I don't have signal-gnd-signal-gnd-signal pattern on my idc, its all signal-signal-signal mode.

For this issue, I tried replacing the IDC cable with standard DuPont style female-to-female header cables covered with aluminum foil that is grounded. This improves the signal integrity significantly and I am able to see the data transfers successfully.

  1. Can you suggest what alternative cables I could try to use to be compatible with the standard IDC socket (or 2x8 header DuPont style) that has shielding that I could potentially attempt to make? I am trying to avoid redesigning the board, sockets and cabling. Any suggestions and solutions would be deeply appreciated.
  2. These wires from https://www.dreamsourcelab.com/shop/accessories/shielded-fly-wires/ seems to be of ideal dimension and seems to be shielded, can we make a 2x8 header from these? What are these wires called and where can I source them?
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  • \$\begingroup\$ How long are your cables? I have a hard time believing this is a shielding issue if you don't even get the signals. "Back when I was a kid" we used long unshielded 40 pin cables at 16 MHz for hard drives inside a computer packed with other equipment. \$\endgroup\$
    – pipe
    Nov 22, 2022 at 1:23
  • \$\begingroup\$ My cables aren't that long. Less than 10 inches. But tdm lines run at 25 mhz and spi lines run at 1 mhz \$\endgroup\$
    – sandeepzgk
    Nov 22, 2022 at 3:09
  • \$\begingroup\$ True that's what I thought about idc cables and the reason for chosing it in the design. But apparently I seem to get better signal when I tin foil the wires :( \$\endgroup\$
    – sandeepzgk
    Nov 22, 2022 at 3:11

1 Answer 1

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I'd suggest your issue is the path for the return currents.

You might like to think of just the signal wires, but for every single-ended signal you drive, there has to be a return path.

If there are no low impedance ground wires near by (remembering that impedance here is tied to the loop area between your signal and return wire), you will tend to get significant cross-coupling of signals. The large loops will also be perfect for coupling external RFI.

There are really just three solutions, all involve reducing loop areas:

  • Have lots of ground wires through the width of the cable, ideally alternating signal and ground pins (the classic setup for old IDE cables)
  • Use differential drivers for your cable links, with the +/- signals on adjacent pins (converting to differential for the link, then back again at the other end)
  • Have an parallel conductor (like your foil) which is connected to ground at both ends, ideally at multiple points, through a custom connector.

The reason why the last option requires a custom connector, is that most user of IDC cables opt for one of the first two options...

If you really can't change your overall pin assignment, I'd suggest you make your own custom IDC cable with an integrated ground plane. This is fairly easy to do using self-adhesive copper tape running length wise on one side of the ribbon cable. At each end solder on a couple of short wires with single socket 2.54mm connectors.
Tweak your design to include suitably located single pin headers connected directly to your PCB ground planes, and when you insert your cables into the IDC header, use these pins to connect the ground fly-wires.

I strongly suspect this will give you much better signal integrity, as the loop area for each signal is now very significantly reduced...

You might even be able to get away with just one wire to the ground strip, located in the middle at each end, as this still greatly reduces the loop area. And instead of adding header pins, you could trying soldering a short flying wire to either your top ground plane (if you have one) of the negative of a near by decoupling cap.

If you do use differential signals or alternating signal-ground, the "ultimate" ribbon cable is "Spectra Strip" which twists together the signal and ground pairs for most of the length, with regular untwisted lengths for adding IDC connectors. Great stuff when you need extra integrity, but a little expensive.

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  • \$\begingroup\$ If you give this a go, and it works, please remember to mark the question as answered, as this will ensure other who have similar issues know what to do. \$\endgroup\$
    – colintd
    Feb 21, 2023 at 12:47

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