Computer front panel USB not grounded - Page 1
Forum section: electronics / beginners

author: fubar.gr
author_rank: Unknown
timestamp: March 11, 2015, 07:21:17 pm
content: In rear USB sockets the metal shield is grounded. However this doesn't seem to be the case with most (all?) front panel USB sockets.In all front panel boards I've seen so far, the shields are connected to a big plane on the PCB, but this plane is floating. And this plane isn't extending to the mounting screws, so grounding via the chasis isn't possible either.There have been several cases reported where connecting a statically charged USB device led to the computer hanging up. This would be avoided if the USB shield was grounded.Is there a reason for this design practice?

author: jmole
author_rank: Regular Contributor
timestamp: March 11, 2015, 07:55:11 pm
content: To add to this, most well made devices (B end of the cable) will connect the shield to ground through a large RC filter.On the front-panel USB, I'd imagine they are trying to use the case for the shield ground connection, although if you have a plastic case, this obviously wont work. You don't want significant current running in the shield, but you want it at the same potential as your chassis ground.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 11, 2015, 08:16:36 pm
content: My front panel connectors are grounded to both the chassis and the USB header ground. Checked 4 different computers with completely different cases.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 11, 2015, 08:23:08 pm
content: Did you check with an ohmmeter? Sounds like the big plane should be a ground plane and I'm surprised that's not connected to chassis.

author: electr_peter
author_rank: Unknown
timestamp: March 11, 2015, 10:35:49 pm
content: Front panel USB may have only 4 separate wires coming from 2.54mm pins on motherboard, so shield may be missing/not connected. I think it could be designed in such way because it is shielded by frame, it is not the end of the world, and most importantly, connecting 4 loose wires to 4 pins is much easier during assembly than 5 to 5 (just think of amount of possible incorrect 5 to 5 connections).

author: fubar.gr
author_rank: Unknown
timestamp: March 12, 2015, 04:20:09 pm
content: Some photos of the board. It has two USB and one Firewire portThe big ground plane is connected to the two pins in the middle, but there's no wire there so it is simply floating (I double checked with a multimeter).The plane extends to one of the screw holes, but is is silkscreened so it doesn't make electrical contact with the screw.

author: SeanB
author_rank: Super Contributor
timestamp: March 12, 2015, 04:38:29 pm
content: Just solder a wire to it and connect to case then.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 12, 2015, 04:41:28 pm
content: Probably just lazy/incompetent case design. They reused the PCB from another case, and didn't consider the effect on grounding, or decided to save $0.02 by omitting a ground wire.Soldering a wire to the case metal is probably the easiest way to fix it.

author: katzohki
author_rank: Frequent Contributor
timestamp: March 12, 2015, 06:01:50 pm
content: Ah-hah, yep! They cheaped out by not connecting to ground!

author: workwithme
author_rank: Newbie
timestamp: March 12, 2015, 08:47:43 pm
content: That's just odd design, hahahaha.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 12, 2015, 10:43:48 pm
content: The shield and signal ground should be tied together with a low impedence path on the board. Why they didn't do this, I have no idea. It wouldn't have cost anything to connect the ground plane to both.http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf

author: ivan747
author_rank: Super Contributor
timestamp: March 13, 2015, 03:06:01 am
content: Quote from: fubar.gr on March 12, 2015, 04:20:09 pmSadly, that PCB layout is crap, it's not surprising that they didn't connect the shield to chasis ground. Do you see the 2 central pins on the USB ports? Those are the data lines, they are supposed to have the same length, so that the differential signal arrives at the same time (google "usb matched pair"). They didn't care about that, they didn't even made an effort to kind of do it a little bit. It almost looks like they didn't even know what they were doing. They didn't care about that in the firewire port either, where it's probably even more critical, as firewire is usually faster than USB 2.0.My recommendation is that you ground it yourself, as other suggested. Maybe next time switch brands or find a case that has more attention to detail.For more info on matched pairs on USB:http://www.ti.com.cn/cn/lit/an/spraar7c/spraar7c.pdf