Custom USB PC-PC Interface

The impedance of the data line is determined by

• capacitance between wires and
• inductance of the pair of wires.

It's a simple formula and I've used it to make custom cables for high speed data links in circumstances where the wires and overall covering needed to be made from PTFE material to suit high temperature applications. The formula is

Zo = sqrt(L/C) - this is the impedance for a lossless line. Here is a good article.

This tells you that to make a higher impedance cable L needs to grow and/or C needs to reduce. One of the methods is to make the actual conductor a smaller diameter whilst keeping the spacing between conductors the same. Or you can make the spacing between conductors bigger. Below is a picture of an on-line calculator: -

Note that the formula given on the page above (Zo) is exactly the same as the one I gave earlier but it uses the more fundamental properties of wires to give L and C. In other words, permeability and permittivity are built into the equations but the 2nd formula explicity lists permittivity because that will be a variable due to the insulation type.

And here is the link to the webpage for the calculator. Please note that there are several calculators on-line so you can double check other ones.

From memory USB cable impedance is 90 ohms.

And if all this seems to much trouble, just make the wires thinner and space them apart a bit more and see what happens. If it is a Zo issue then things should begin to improve as you do this.

Some consumer electronic devices have occasionally used 3.5mm TRRS to USB connectors. For instance, the LG A155 mobile phone and several older MP3 players do this.

The most commonly used pin-out (iPod Shuffle 2G, Dolphin Swimmer MP3 player, Samsung YP-F1) is (source):

• Tip --> D-
• Ring1 --> D+
• Ring2 --> GND
• Shield --> VUSB

This is, however, not defined in any official standards document to my knowledge. Thus, calling it "usual practice" might be stretching things a bit.

From a short-circuit prevention perspective, moving away from the practice listed above has advantages:

Having VUSB and GND next to each other may cause an accidental short from power to ground during insertion or removal of the TRRS plug.

However, since D+ and D- ought to be in a single twisted pair in the USB cable to minimize common-mode noise pick-up, this segregation must be done at the TRRS connector, not at the other (USB connector) end of the cable.

Note that using a TRRS connector will only work for 4-contact USB, and will not propagate the USB ID line.