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USB Trigger oddity

Question asked by chrismower on Jul 16, 2015
I was doing some USB data capture work and was using the normal edge trigger since I couldn't get the scope to trigger when set to Type->USB Trigger On->SOP but it was proving to be hard work. Eventually I found that there seems to be an error in the D+/D- setting for the trigger in that they are the wrong way round. It is my understanding that for Full Speed (12MBPS) USB the D+ line is high signalling low, and the D- line is low signalling high, which is how I had set the scope up, with Channel 1 connected to D+ and Channel 2 connected to D-. When I reversed the scope settings it suddenly started to trigger.

I have re-checked my understanding of the USB signalling, along with the scope connections, and can only deduce that there is an error in the scope settings. I have attached a screen cap of the scope showing the reversed trigger settings along with the Wikipedia entry. I am happy to accept that there is an error in my understanding, but for the life of me I can't see what it could be. Can someone at Keysight comment?

Note: Added - The scope in question is an MSO-X 3012A. The USB connection is between an iMac and an Arduino UNO. Firmware version is 02.39.2015022602

Edited by: ChrisMower on Jul 16, 2015 6:59 AM  

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