drain wire in USB 3.0 (GND_DRAIN) - not a "signal return path" - Page 1
Forum section: electronics / beginners

author: smaroukis
author_rank: Newbie
timestamp: November 09, 2023, 04:08:27 am
content: Isn't it incorrect to say the drain wire in USB3.0 is a "signal return path" for a differential pair, as quoted on the USB3.0 wikipedia article under Pinouts.https://en.wikipedia.org/wiki/USB_3.0> GND_DRAIN Ground for signal returnAs I understand the bare drain wire is included in shielded conductors to provide a better conductive path for EMI surges that the thin aluminum shield could not handle.Technically the return path of the signal will be wherever the "board ground" is. The GND_DRAIN should be connected to the shield, and thus the chassis ground, which may or may not be connected to the board ground.Picture showing board ground versus chassis ground to make sure we're on the same page:https://www.quora.com/Is-the-USB-metal-connector-shorted-to-negative-GND-pin-1

author: ataradov
author_rank: Super Contributor
timestamp: November 09, 2023, 04:23:41 am
content: There are 3 shields in the USB3 cable. One is common outer shield and then there is an individual shield for SSTX/SSRX pairs. Those two shields are connected to the GND_DRAIN and it must connect to the hard board ground. The later two are not necessarily fully shielded, they may just have a separate plastic insulation. But in any case, there is a separate solid wire in each of them.The shield one is separate and should go though the RC filters. Although there are different opinions on that at least for USB2 devices.

author: smaroukis
author_rank: Newbie
timestamp: November 09, 2023, 07:19:48 am
content: So I see you say to connect the GND_Drain (connected to the two shields and drain wires for SSTX/SSRX) directly to the hard board ground. Why not connected directly to the shield (and then decide what you want to do with the shield, either directly to board ground or through a filter - I also saw the differing opinions on this).In usb-c it seems we have the same shield wires for the Tx/Rx, but we don't have a GND_DRAIN pin, so I'm assuming in that case the shield wires are connected to the shield directly.I guess my question comes more down to is, are the solid wires for SSTX/SSRX for EMI surge dissipation (as I thought was the case for a "drain wire"), or are they for a return path for unbalanced differential signals?

author: gcewing
author_rank: Regular Contributor
timestamp: November 09, 2023, 07:38:31 am
content: Some places in the USB protocol use single-ended signalling on the differential pairs, i.e. both wires driven the same way, so a ground wire of some kind is needed to interpret those signals with respect to.

author: ataradov
author_rank: Super Contributor
timestamp: November 09, 2023, 03:16:31 pm
content: Quote from: smaroukis on November 09, 2023, 07:19:48 amBecause it is the actual signal ground, it is directly related to the signal integrity.Quote from: smaroukis on November 09, 2023, 07:19:48 amIn the Type-C connector those are connected to the GND pins, which are specifically located close to the RX/TX pairs.Quote from: smaroukis on November 09, 2023, 07:19:48 amThey are a common reference for the differential signals, I don't think they are just for EMI.

author: wraper
author_rank: Unknown
timestamp: November 09, 2023, 03:49:05 pm
content: Quote from: ataradov on November 09, 2023, 04:23:41 amIt must be either connected directly to grounded enclosure or directly to PCB GND. Adding random parts between the shield and GND only makes connection unstable and causes EMI issues.https://www.unit3compliance.co.uk/hdmi-more-like-hdm-why-thoughts-on-cable-shield-grounding/

author: ataradov
author_rank: Super Contributor
timestamp: November 09, 2023, 03:59:24 pm
content: I personally agree with that. But the spec is vague and says that you need to do what is best for EMI/EMC in a specific situation without actually providing any real advice. I've seen no evidence that using RC filters improves EMC in any way, but there may be cases where it does.

author: wraper
author_rank: Unknown
timestamp: November 09, 2023, 07:25:13 pm
content: There is not just RC, there is LC, ferrite beads, in series, in parallel, totally random stuff with random component values. All sorts of random Voodoo with zero understanding what and how it's supposed to achieve. All because of fighting with mysterious ground loops or whatever. Often combined with "ESD protection" in a way that instead of actually protecting anything it diverts ESD discharge from the shield directly into data lines, I'm talking about original Arduino UNO for example.

author: smaroukis
author_rank: Newbie
timestamp: November 11, 2023, 02:48:56 am
content: Got it, so my understanding of a drain wire was wrong, probably because I read about it not related to high speed differential pairs (more for EMI discharge on industrial equipment)Here's my understanding now:> We want to provide a single signal ground return closest to the signal pair to minimize the transmission line and return loop area. If the signal is perfectly balanced and differential, there shouldn't be a return signal (signals perfectly cancel at the receiver), but in the case of slightly unbalanced lines or in the case we use the differential pair as a single ended signal, for high speed cases we want this signal return as close as possible to the original signal.Also, this drain wire should be connected to the hard board ground as close as possible to the Tx/Rx pairs for the same reason. (and actually this is how it is done inside the cable on a USB-C using shielded twisted pairs - as opposed to coax which uses the coax shield as the signal ground).For the shield, also connect that directly to hard board ground, unless you have a specific reason to add something in between for your EMC use case.