USB cable shield question

[Red Squirrel]

Is the shield in a USB cable the same ground as the black wire, or is it actually a separate ground? Basically, if I fix the shield with foil is it safe to just connect it to the black cable?

 

[mfenn]

Yes, the shield is just connected to ground.

 

[richaron]

Yes, the shield is just connected to ground.

OP, I would do a little research first. Honestly don't like questioning mfenn, but a few quick search results suggest it's the host which grounds the shield (i.e. inside the computer, not inside the cable). Don't know if it makes a diff tho...

 

[ericloewe]

What's your problem anyway? What kind of repairs are you talking about?

Anyway, it might not be as linear as connecting it to the ground wire, since the USB spec says that the shielding must only be grounded on the host. I'd say it should work with ~90% certainty, but you never know.

 

[Red Squirrel]

OP, I would do a little research first. Honestly don't like questioning mfenn, but a few quick search results suggest it's the host which grounds the shield (i.e. inside the computer, not inside the cable). Don't know if it makes a diff tho...

I was playing around with my multimeter and from what I see there must be a resistor or something at both ends of the cable. I was never getting continuity between the shield and the cable ends or the ground. So yeah what I ended up doing is not hooking it up to ground, but I just made sure part of the shield was sticking out and I wrapped it in foil and the pressure keeps it contacting. Chances are I could have gotten away without the foil but figured I'd play it safe.

This is what I did: http://www.redsquirrel.me/2012/06/zalman-zm-ve200-enclosure-power-fix.html

 

[Vinwiesel]

Yes, the shield is just connected to ground.

You sure? I just tested a couple cables and the shield is isolated from the 4 wires. Once you plug it in, the shield is connected to chassis ground, but in the cable itself it should not be connected to GND.

 

[Red Squirrel]

You sure? I just tested a couple cables and the shield is isolated from the 4 wires. Once you plug it in, the shield is connected to chassis ground, but in the cable itself it should not be connected to GND.

Yeah from my testing it is isolated too, and think it makes sense. Think of it as neutral and ground for your home electrical. At the end they are connected, but they are still separate throughout the rest of the system. In the case of USB it's done to isolate any noise so they don't hook it up to the same wire used for active power ground. At least that's my guess.

I ended up having to replice though, I guess the +5v was a tad too close to the shield and depending on how the wire moved it would contact. I had put lot of glue but guess I missed a spot! I decided to omit the glue and just make sure there's electrical tape masking the solder joint, and then more electrical tape wound tightly around the entire cable with tie wraps at both ends to keep it tight. I need to find a place that sells shrinkwrap as it would probably be better for something like this. Just nice to have this drive working now.

 

[Vinwiesel]

Do you have a powered USB hub? Same concept as your power splice as it will provide power to the drive regardless of what the computer can supply. Front panel usb plugs are often crummy for external hard drives as well since they are basically an extension cable, which is bad for data and power.

 

[mfenn]

OP, I would do a little research first. Honestly don't like questioning mfenn, but a few quick search results suggest it's the host which grounds the shield (i.e. inside the computer, not inside the cable). Don't know if it makes a diff tho...

That's what I said, though I see how I said it could have been confusing. The black wire is reference voltage, which should be close to ground, but isn't necessarily exactly the same.