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?
Best Answer
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.