Probing USB signals using an USB Logic Analyzer

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Hello,

I was just wondering If it is safe to probe USB data lines using an USB Logic analyzer(The 10$ ones). I have this question because USB have differential signalling.

Thanks,

Sandeep

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Last Edited: Tue. Nov 23, 2021 - 12:58 AM
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Well; You might use Sigrok software with your $10 Analyzer. It's a common recommendation in many forums.

In Sigrok, there exists a USB-LS and USB-FS decoder (https://sigrok.org/wiki/Protocol_decoders), therefore I would guess that it is perfectly safe.

 

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"Dare to be naïve." - Buckminster Fuller

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It's perfectly safe, the differential signalling is referenced to ground. It's not there for isolation, it's for noise immunity and to reduce emissions.

 

I've done it a lot, made up a few cables with exposed wires because I'm too lazy to make a PCB :)

 

The only issue you will have with a $10 logic analyser is that it's too slow for even USB Full Speed. Low Speed will be okay but Full Speed is 12MHz, and they only go up to 24MHz so will incorrectly read some bits due to the analyser's clock being out of sync with the USB one. In my experience Sigrok has great USB decoding but it might take you half a dozen captures to get the bit you are interested in to decode properly.

 

I've actually been looking at faster LAs that are supported by Sigrok, but the ones that support at least 48MHz needed for USB and are cheap are a bit thin on the ground. There is the DSLogic and various Kingst models and OEMed devices. There's a list here, but most of them are no longer available: https://sigrok.org/wiki/Supporte...

 

The other option is get a better oscilloscope that is supported by Sigrok and which has digital channels. They tend to be rather expensive though, the  Siglent SDS2000 being about the cheapest option but the wiki is kinda vague about how well it actually works with Sigrok.

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"Dare to be naïve." - Buckminster Fuller

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Thank you. This is exactly what I wanted to know. yes

Last Edited: Tue. Nov 23, 2021 - 12:59 AM
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epyczen wrote:

Thank you. This is exactly what I wanted to know

Why didn’t used another USB as USB analyzer ? Cost 1 US$

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