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Electronics => Beginners => Topic started by: ssashton on January 11, 2023, 12:14:59 am

Title: What is the meaning of USB DM?
Post by: ssashton on January 11, 2023, 12:14:59 am

Simple question but I couldn't find it in google.

In USB schematics D+ Data Positive is called DP. Why is D- Data negative called DM and not DN?

While I'm asking, can anyone help explain what the other USB OTG connections do? Such as OTG_HS_ULPI_NXT and OTG_HS_ULPI_DIR?

Thanks!

Title: Re: What is the meaning of USB DM?
Post by: BillyO on January 11, 2023, 12:50:45 am

The data pair in USB is differential.  It is just normal practice to call the (-) side Data Minus (DM) and the (+) side Data Plus (DP).

For OTG see: https://en.wikipedia.org/wiki/USB_On-The-Go (https://en.wikipedia.org/wiki/USB_On-The-Go)

Title: Re: What is the meaning of USB DM?
Post by: tooki on January 11, 2023, 07:01:12 pm

I have mostly seen it called DN in schematics I’ve seen.  :-//

Title: Re: What is the meaning of USB DM?
Post by: Rick Law on January 11, 2023, 10:22:44 pm

Interesting how minds work:
Opposite of Positive is Negative (DN)
Opposite of Plus is Minus (DM)

DN is probably more natural to those more comfortable with English.  Non frequent speaker of English see the + on electricals and in their minds translates it to their native language to whatever means electrically positive.  The opposite of "electrically +" in their native language could be rather different than the word "negative" when translated back to English.

The + and - is less language dependent and just a math symbol.  Personally, I think D+ and D- is probably more suitable for international.

Title: Re: What is the meaning of USB DM?
Post by: mwb1100 on January 11, 2023, 10:48:13 pm

FWIW:

  - the USB 2.0 spec (https://www.usb.org/sites/default/files/usb_20_20211008.zip) uses D+ and D-
  - the USB 3.0 spec (https://usb.org/sites/default/files/USB%20Type-C%202.2%20Release%20202210%20%281%29.zip) uses Dp1 and Dn1 (and Dp2/Dn2 for the doppelgangers on the USB-C "flippable" connector)

So even they couldn't make up their minds.  Though the change is understandable; I think "D+1", "D-1" (etc.) would have been asking for confusion.

Title: Re: What is the meaning of USB DM?
Post by: tooki on January 12, 2023, 07:50:02 am

Quote from: Rick Law on January 11, 2023, 10:22:44 pm

The + and - is less language dependent and just a math symbol.  Personally, I think D+ and D- is probably more suitable for international.

And it is analogous to the inputs on an op-amp.

One problem is that in some PCB layout programs, you can’t use + and - in net names (or not in the differential pair function). For example, in Altium, a differential pair’s nets must be named, for an example pair called “D”, exactly D_P and D_N. Anything else and you break the pair apart. (IIRC, this limitation may have been remedied in a recent update.)

Title: Re: What is the meaning of USB DM?
Post by: ssashton on January 17, 2023, 11:21:10 pm

Thanks for all the replies!