USB communication not differential? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2016, 08:29:15 am
content: AFAIK, USB communication is differential. ie when D+ is HI, D- is certainly LO (when sending data, not termination "end of data"). is this set of mind correct? because if it is, i will report the Rigol DS1000Z long memory capture BUG to them. here are some snapshots of the USB data captured.1) USB should looks like this right? differential?2) sometime i'm capturing D+ moving around only.3) sometime i'm capturing D- moving around only.is this non-differential type nature (snapshot #2 and #3) of USB communication normal? but i think its not.ignore the ~100ns CH1 delay in the pictures, i'm going to report that bug anyway to Rigol...

author: ataradov
author_rank: Super Contributor
timestamp: March 25, 2016, 08:46:56 am
content: There are cases when USB is used in single-ended mode, but I don't think it ever happens for large chunks of data. It is mostly used to indicate various device states, not communicate data.On the other hand, it would be a very strange bug.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2016, 09:21:02 am
content: i'm also detecting such kind of data when captured from DS115E, i think this is normal USB comm, yes it will be strange bug when dso stop or miss capturing in the middle of data/memory. i hope no bug here i just wish more confirmation from the community that this is normal. the device probed is Hantek DDS-3x25. edit: here is the snapshot from DS1152E. mixed differential and somekind of duplex (2 way independent) communication?

author: fubar.gr
author_rank: Unknown
timestamp: March 25, 2016, 12:04:10 pm
content: Quote from: Mechatrommer on March 25, 2016, 08:29:15 amAll electrical signals are differential.When the term "differential" is used in the context of USB, Ethernet and other high speed comms., what is really meant is that the signal is isolated from ground. And since nothing can be trully isolated or "floating" what is really meant in effect is that both wires of the pair have a very high impedance relative to the ground.You can think of the D+ and D- pair like the output of an isolation transformer. So you can't really say that when D+ is high, D- is low. They are high and low relative to each other and not relative to earth ground or some other reference point.So you assume that one wire is the "ground" point (usually D-) and the other one carries the signalAnd you can't probe with an ordinary grounded oscilloscope probe, because you are messing up the isolation of the pair. You need a special differential probe

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2016, 01:57:42 pm
content: Quote from: fubar.gr on March 25, 2016, 12:04:10 pmno i didnt assume or said that i assume that... both D+ and D- are connected to CH1 and CH2 respectively through the hi-Z probe tip. neither D+ nor D- is connected to the DSO BNC's GND shielding.Quote from: fubar.gr on March 25, 2016, 12:04:10 pmwhat i meant is that... the difference between both D+ and D- (relative to each other) is either -V or +V, but never 0V (except in some special conditionsfor a short duration such as "end of data" termination protocol or what not? i havent read usb bluebook). in normal idle or data transmission, they should be "relatively"... either +V or -V only where |V| is usually = 3.3Vpk, correct? this was my question.Quote from: fubar.gr on March 25, 2016, 12:04:10 pmexactly! exactly! thats what i was refering to...Quote from: fubar.gr on March 25, 2016, 12:04:10 pmmaybe i wasnt clear enough. let me rephrase... when D+ is HI, D- is certainly LOrelative to D+and the absolute |magnitude| of the difference is somewhere 3.3V.Quote from: fubar.gr on March 25, 2016, 12:04:10 pmi believe my probe setup is the advisable method here and there, the poorman 2 channel dso probe. its just i didnt turn math ON, i did the subtraction in my head.So, the same question again but in different way... can the usb comm state or difference between D+ and D-, is so frequently happen to be 0V? like my picture #2 and #3 above? is there a hard proof specification for duplex, single ended comm for usb and are they so common happening in real life?

author: jeroen74
author_rank: Frequent Contributor
timestamp: March 25, 2016, 02:03:02 pm
content: USB does use single ended zeros, apart from the normal differential ones and zeros.http://www.usbmadesimple.co.uk/ums_3.htm

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 25, 2016, 03:22:03 pm
content: thanks for the link, thats look really like "usb made easy to read". i saved all of them for later reference.

author: ade
author_rank: Unknown
timestamp: March 25, 2016, 07:47:00 pm
content: I only have superficial knowledge of USB so please bear with my ignorant questions...The capture seems to imply both 0s and 1s are being sent single-ended, but I thought SE1 is illegal? You can see all combinations of D+ and D-: 00, 01, 10, 11.Also I thought SE0 is only used for a couple bits worth of time here and there like at the end of a packet, before going back to J-state?