USB extension interference

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USB extension interference

Postby pjpkwy » Mon Dec 03, 2012 8:35 pm

I've built myself a roll around workstation out of a craftsman SS side box with a small desktop pc with surge/battery back up in the botom drawer and mounted it on four casters and have a section of strut bar exiting the top with the monitor and keyboard mounted to it.

I then connected a usb extension(about 3 ft) to the pc and ran it up through the bar to make removal of the pico easier in the event I had to connect to my laptop and do a road test. When I turn on the pico 4423 and have test leads connected, I get a ton of noise as it's set to auto. If I move to the 1 volt or higher it isn't as bad. If I touch the leads together(zero) I get a straight line but with dashes of noise along it. This happens on any channel I happen to have test leads connected to. If I connect the pico directly to the pc(bypassing the extension), it's clean and clear. I then went with a usb 4 port hub with its own external power supply with the same results as the extension.

I'm sensitive to this because about two months ago I was using my pico to show a customer the difference between her newly replaced O2 (from another shop) and the one that actually needed replacing for her original code. That afternoon I went to do vacuum pulls on a no start and when I hooked up my bnc cable to channel B I had all kinds of noise that looked like vacuum pulls, but I had yet to hook up the transducer to the other end. If I disconnected the cable it would flat line, rehooking brought back the noise as did hooking up regular test leads. this only happened on channel B. I sent it back to AESwave who then sent it back to pico for repair and I was informed that different grounds were connected causing a "ground loop" to the unit and current flowed from one negative point to the other and POOF. Not sure what exactly a ground loop was, but given his explanation from AES. I don't ever remember making such connections, not to mention it worked fine on the previous job.

Sorry so wordy, but wanted to be as descriptive as possible and I contacted Pico with this and haven't heard back in over a week and I don't want to risk damaging my Pico as I stated before, I don't recall doing anything wrong in the way of connections that would have caused the first failure.

Just wanted to know is it a cable quality issue or the length of the connection, etc.?
pjpkwy
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Re: USB extension interference

Postby AuthorMike » Tue Dec 04, 2012 8:53 am

Hi There,

Thanks for your query. I hope you don't mind but I broke your post up a bit (just to make it easier on the eyes).

I believe this has been looked into and a response now sent?

Please let me know if you have not received anything.
Mike Valentine
Technical Author @ Pico Technology UK
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Re: USB extension interference

Postby Martyn » Tue Dec 04, 2012 8:58 am

The following response was sent by a colleague through our helpdesk system on the 22nd Novemeber

If you want to split the USB feed from the PC to the Scope you really need to:-

1. Use our new "Blue" USB leads for both halves of the run. (They are of a different design to a normal lead).
2. Use a powered USB Hub to connect the two leads together, especially if one of the leads is longer than the standard 1.8 metres.

We sell Blue leads in lengths of 1.8 metres & 4.5 metres:-
http://www.picotech.com/accessories.html#cables

I cannot trace the repair for your scope, you gave too few details, so I cannot see what fault was found.


Is it possible that your email system treated this as spam ?
Martyn
Technical Support Manager
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Re: USB extension interference

Postby pjpkwy » Tue Dec 04, 2012 2:57 pm

Martyn, Thank you for getting back to me so quickly. I read the excerpt from Richards response as it was in my junk folder. I did try suggestion 2. but still have the same issue so I'm going to have to get Pico's cable which is fine, I just wanted to make sure I wasn't doing damage to the unit. It looks like it was also part of Richards response in the excerpt that the repair could not be traced/tracked because of to little information I supplied, but I did not see it in his original response that was in my junk folder, but he did explain the " ground loop". I have no recollection of ever hooking up something like that. My AESwave invoice shows my serial number as AR584/099. Thanks again, PJ.
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