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USB lead length problem

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USB lead length problem

Postby Bob Mehew » Wed Jan 20, 2016 10:27 pm

Just had a dreadful day when I thought my Pico 4424 was broken. I went out to do some work in an unheated environment and realized I had forgotten to take the supplied USB lead with me. So I found a 0.63m long one and plugged that in. With no BNC connector plugged in, the notional zero read +15mV on the -50 / +50mv range and 0.45v on the -2 / +2v range. Worse still when I tried to input a signal to the scope on -2 / +2v range, it refused to show values above 0.45v. (I confirmed this with voltmeter and a battery.) Thinking it might be condensation (the snow was crisp) I tried warming the Pico up but with no success. So came home and tried again on a range of USB leads. It appears that both a 06.3m and a 0.53m long USB lead cause the same problem even though they are OK for other uses. Other USB leads over 1m long were found OK. But the 0.53m plus a 2.5m USB extension lead was not OK. Has anyone an explanation for this observation? I must admit I am just grateful I only wasted 200 miles of driving and am not faced with a faulty Pico. :D
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Re: USB lead length problem

Postby Hitesh » Wed Jan 27, 2016 9:36 am

Hi Bob,

It could be down to the way the USB cables have been manufactured.

It is recommended to use the blue 'Pico' USB cables that we provide with the products as they are designed to provide sufficient power for the devices and have screening. The screening is connected to ground at the PC end.

Regards,
Hitesh

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Re: USB lead length problem

Postby Bob Mehew » Thu Jan 28, 2016 4:45 pm

I thought all USB cables were required by the spec to be shielded (indeed doubly so with copper braid plus aluminumised polyester foil). However, I thought the experience was worth posting as a 'cautionary tale'.
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