Author Topic: Modding an optical USB cable to be isolated  (Read 233 times)

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Offline Mr Evil

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Modding an optical USB cable to be isolated
« on: April 02, 2021, 02:00:50 pm »
I need to connect a USB cable from my PC to my oscilloscope on the other side of the room. This exceeds the distance that normal passive USB 3 cables can work reliably at (2m, or somewhat more with a very good cable), so I looked into optical cables.

Optical cables will work over very long distances, and since they don't need an electrical connection between the two ends for data, they could be isolated. I don't require isolation, but it would be handy to have. However, I couldn't find any existing products that were definitely isolated. The closest I could find was one made by EB-LINK which requires the device end to have a separate power connection, which I hoped meant that it didn't have any wires connecting ground and 5V (other cables use external power too, but often say that they might get away without it if the device doesn't need much current, implying that there is an electrical connection between host and device).

So I bought a 10m long one, and it looks like this:


A USB cable powered by USB! Sadly, there was continuity between the ground pins. That wasn't satisfactory, so I immediately disassembled the very expensive cable, to reveal this:


You can just barely see the optic fibres going into the optical thingies on each PCB. On the other side of the PCBs are two wires labelled GND and +12V. As I've come to expect from Chinese goods, GND is red, and +12V is black. The host end also has two unused pads for D+ and D- (maybe for the missing USB 2 support - this cable only works in USB 3 mode). 12V is an odd voltage for a USB cable, so I took some measurements, and there was no voltage present on it at all. Indeed, it looked like the host end was powered only by the host connection, and the device end was powered only by the micro-USB port on that end.

So I simply cut the wires. I chose to do so on the host end, as the cable on that end has a metal clamp over it, whereas the device end is held only by glue and the strain relief. The result looks like this:


I had to be very careful, as the joints had a blob of some sort of plasticky substance covering them, making them hard to desolder, and the optic fibres are very thin and breakable. I managed to get it all done and reassembled without damage though, and... it works fine.

The wires still pass into the metal shells of the connectors, so the isolation voltage will be whatever the insulation on the cable can withstand.

Why are those two wires present in the first place if they aren't necessary? Maybe they are for some feature that isn't enabled in this particular cable, or maybe they are purely for mechanical support.

Offline tonyh88

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Re: Modding an optical USB cable to be isolated
« Reply #1 on: April 02, 2021, 03:50:43 pm »
I can't answer you question about the extra cables But I just wanted to point out that USB extenders are definitely a  thing. I would assume that anything that is over optical fiber will be isolated as well but hat would need to be checked in the datasheets of the products.

Optical fiber, ethernet etc...

Some examples, Most of the time they will require a power supply of some sort on both side but some of them powers directly from the usb or only needs power on one side :

« Last Edit: April 02, 2021, 03:57:56 pm by tonyh88 »

Offline Mr Evil

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Re: Modding an optical USB cable to be isolated
« Reply #2 on: April 03, 2021, 12:33:18 pm »
I like the idea of using standard network cables, but those adapters + cable would end up significantly more expensive.

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