USB D+ D- identification - Page 1
Forum section: electronics / repair

author: electr_peter
author_rank: Unknown
timestamp: April 27, 2015, 10:00:05 pm
content: Hi,I have laptop with weird USB connector and I plan to add standard connector. I presume USB port functions well, but I do not know exact pin out. 5V and GND is known.Is there a way to discriminate between D+ and D- pins just by checking voltages? Or is it a question of functional test?

author: wraper
author_rank: Unknown
timestamp: April 27, 2015, 10:18:58 pm
content: Quote from: electr_peter on April 27, 2015, 10:00:05 pmWhat is weird in it? Why would you want to add "standard" connector. Are you sure that it is USB at all?

author: wraper
author_rank: Unknown
timestamp: April 27, 2015, 10:32:06 pm
content: As a guess, "weird USB" could be 4 pin IEEE 1394 (firewire) considering the same pin count.http://en.wikipedia.org/wiki/IEEE_1394

author: SL4P
author_rank: Super Contributor
timestamp: April 28, 2015, 12:06:25 am
content: --- or HDMI / DisplayPort ?

author: electr_peter
author_rank: Unknown
timestamp: April 28, 2015, 07:55:54 am
content: USB connection is on the LCD display of ThinkPad X2x series. It is definitely USB (manual and available adapters (cameras, mics) confirms it. There also was simple USB adapter for these devices). Official name is "ThinkPad UltraPort".

author: SL4P
author_rank: Super Contributor
timestamp: April 28, 2015, 07:57:42 am
content: My X220T has USB & USB 2.0 - and a mini-displayport connector...

author: Rasz
author_rank: Super Contributor
timestamp: April 28, 2015, 08:28:27 am
content: Quote from: blueskull on April 28, 2015, 12:13:46 amwait what? rethink that statement

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 28, 2015, 08:47:33 am
content: http://www.thinkwiki.org/wiki/UltraPort

author: rs20
author_rank: Super Contributor
timestamp: April 28, 2015, 01:07:13 pm
content: People, the OP is asking how to disambiguate between D- and D+. If you've got two wires in your hand, D+ and D-, and you don't know which one's which, how do you figure it out?I think the easiest way is to just try plugging a device in. I'm pretty sure it's safe to do because it's a differential line, so the 1's and 0's will be backwards, no risk of bad things happening (like if you were to get the power and ground backwards).

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 28, 2015, 01:12:09 pm
content: Quote from: rs20 on April 28, 2015, 01:07:13 pmI know, but he's trying to figure this out for an Ultraport. The link I posted shows the connections:

author: Rasz
author_rank: Super Contributor
timestamp: April 28, 2015, 03:01:31 pm
content: Quote from: blueskull on April 28, 2015, 08:38:38 amwtf

author: zapta
author_rank: Super Contributor
timestamp: April 28, 2015, 03:42:33 pm
content: Quote from: Rasz on April 28, 2015, 03:01:31 pmhttp://en.wikipedia.org/wiki/ESATAp

author: Rasz
author_rank: Super Contributor
timestamp: April 28, 2015, 04:07:50 pm
content: Quote from: zapta on April 28, 2015, 03:42:33 pmyes, I was the one eating it

author: electr_peter
author_rank: Unknown
timestamp: April 28, 2015, 04:21:37 pm
content: Thanks for replies, linked schematic seems to match pinout.As for D+ and D- differentiation on host side, it looks impossible to do just by electronic characteristics. Device pulls one of the lines high/low to indicate speed, host just presents differential line.

author: Lukas
author_rank: Frequent Contributor
timestamp: April 28, 2015, 08:48:47 pm
content: I'm wondering why USB isn't capable of swapping D+/D- when necessary, would make PCB layout easier. Newer serial interfaces like PCIe automatically handle polarity inversion.

author: rs20
author_rank: Super Contributor
timestamp: April 29, 2015, 01:21:16 am
content: Quote from: Lukas on April 28, 2015, 08:48:47 pmPCIe also handles crazy stuff like deskewing between different pairs, etc. So once you've got all that processing and buffering in place, adding a not gate is clearly not that difficult comparatively. Also, making PCB layout easier is genuinely an actual issue when you're routing 16 pairs or more. That seems legitimately useful somehow.USB, however, is much simpler and designed to be cheap to implement? Flipping the two lines is just a single via, it doesn't seem that bad. I dunno, I'm just making stuff up here.