2
\$\begingroup\$

I'm using 2 USB hubs chained together to hook up my phone to the computer to do some testing stuff. How can I check if the USB signal is being good or not? I tried scoping the USB pins with the USB hubs connected and disconnected. Both cases had the Data+ pin around 500 mV, is this good? The waveform looks slightly different, with the signal with the hubs connected having more spikes.

Can someone refer me to a good site that covers the USB protocol? How do you know if the signal is good enough?

EDIT: I'm using a Tektronix VM 5000 HD. What voltage range should I expect to see at the Data+ pin?

\$\endgroup\$
  • \$\begingroup\$ What do you actually suspect and what do you want to achieve? Also what sort of equipment do you have? \$\endgroup\$ – AndrejaKo Aug 6 '13 at 16:46
  • \$\begingroup\$ The Wikipedia entry on USB (signaling) is a good place to start. From there, you can probably find some more specific datasheets. Note that USB 2.0 and USB 3.0 are different. \$\endgroup\$ – JYelton Aug 6 '13 at 20:02
4
\$\begingroup\$

There is no "good" voltage for the data lines, particularly when they're not connected. USB doesn't work that way (I'm assuming the voltages are 0 < usb_V < 5V. Anything else would indicate a serious hardware fault, which is outside the domain of your question).

Furthermore, every hub actually receives, interprets and retransmits the USB data signals, so it's not like a bussed-network where all the data wires are tied together anyways. Connecting lots of hubs doesn't "affect the signal integrity" like that, since the signal is completely reconstituted by every hub in the chain.


My guess, assuming these are unpowered USB hubs, is that you're simply exceeding the allowed current draw of the USB host.

Rather then look at the USB data lines, you should look at the USB 5V. It should be within a few hundred millivolts of 5V, and shouldn't have a lot of noise either.

| improve this answer | |
\$\endgroup\$
3
\$\begingroup\$

The "official" way to check USB signal integrity is to put a test pattern onto the wire and look at the eye diagram: http://www.testusb.com/HSEYE.htm

However, (1) as Connor points out the siganl is regenerated at each hub, and (2) you haven't actually said whether it doesn't work or what problem you see. Try with various devices.

There is also a maximum cable length (5m), and according to wikipedia "The USB specification requires that bus-powered hubs may not be connected in series to other bus-powered hubs."

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.