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USB isolator, PicoScope 6 crash

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USB isolator, PicoScope 6 crash

Postby Misan » Mon Apr 18, 2016 10:24 pm

Hi,

I’ve sent a bug report via “Send Feedback” but didn’t receive any confirmation so far, not sure if you got it.

My problem is nearly identical in PicoScope 6.6.50.3 (Win) and 6.11.8.11 (OS X):
When I force the USB - connection (to my 2205) to go down to USB 2.0 Full Speed the scope basically works, but it reproducibly crashes only when taking the collection time above 100 ms/div (that is 200 ms, 1 s, …).
(Rem: My bug report reads “below”, should be “above”, sorry.)
Depending on the demanded collection time the crashes are a bit different.

It seems to be a kind of timeout related to the USB connection.

Basically I want to isolate the GND (= USB GND) of the scope from my PC.
Often the signal ground of test objects has to be connected to power ground for safety. Connecting the probe will produce a ground loop and change the signal, or damage the scope or PC in case of fault.
Using a Laptop could set the casing to mains which is 230 VAC in the EU.

So I’m trying to use a USB - isolator but it’s working in Full Speed (12 Mb/s) only.

Could you please tell me:
a) Did you receive my bug report?
b) Is there any USB - isolator recommended for the PicoScope?

Regards,
Misan
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Re: USB isolator, PicoScope 6 crash

Postby Hitesh » Wed Apr 20, 2016 1:25 pm

Hi Misan,

We have received your bug report and I will also respond via your e-mail support ticket.

Are you using a blue or white USB cable with the PicoScope 2205? The blue USB 2.0 cable that we supply with the PicoScope 2000 Series should provide a more reliable connection. It might be that the different power requirement when switching sampling modes could be causing the issue.

It's also worth installing the latest version of PicoScope 6 for Windows (6.11.12.1692) from our Downloads page.

Unfortunately, we are unable to advise on a USB Isolator. You may wish to look for one with a USB 2.0 connection that can provide sufficient current to the PicoScope device. I would suggest contacting one of our distributors in your country who might be able to help.

Hope this helps.

Regards,
Hitesh

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Re: USB isolator, PicoScope 6 crash

Postby bennog » Wed Apr 20, 2016 5:04 pm

Misan wrote:So I’m trying to use a USB - isolator but it’s working in Full Speed (12 Mb/s) only.


That is USB 1.0 12Mbit/sec == 1.2 MByte/sec.

When your time-base is larger than 200 ms/div the scope is streaming live data to the PC.
So it will only work with sample frequency below 500kSamples / sec (1 channel)
if you want to use 2 channels you it can only be 250kSamples / sec.

If your sampling speed is low enough you can get away with USB 1.0

I will suggest using an USB2 isolator.

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Re: USB isolator, PicoScope 6 crash

Postby Misan » Wed Apr 20, 2016 11:13 pm

OMG! :oops:

Thanks for your replies, Hitesh and bennog!

Yes, I’m using the original blue USB cable from the USB isolator to the scope.
The (cheap) isolator is from Olimex, capable to deliver 750 mA when using an external power supply (I have tried both, a wall plug 12V / 1A and a 12V lead battery to reduce AC coupling to mains).

The crash (connection disruption, scope reset?) happens immediately when changing the time-base, I think before any attempt to switch the relays (a power peak) occurs.

Using Windows, the “Check for Updates” stated “You are using the latest version”, however, today I’ve updated to the 6.11.12.1692

- and this actual version solved the issue under Windows!

No more crash, reset, or whatever, I think it works with the USB isolator (need more time to test, but I’m confident).
Thank you!

So it remains a problem with the OS X version only, maybe you could check that for the next version (I’m on an iMac, would love to run my scope without the Windows - VM).

Yes, a USB 2 would be better, but which one?
Best would be a Pico - Isolator! (why not?)

Regards,
Misan
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Re: USB isolator, PicoScope 6 crash

Postby Hitesh » Mon Apr 25, 2016 1:48 pm

Hi Misan,

I have put forward your suggestion for a USB isolator for consideration.

There should be a new version of the PicoScope for Mac OS X software but I am unable to advise on timescales. The Windows version is likely to be using a different driver version to that used by the Mac OS X version.

Regards,
Hitesh

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Re: USB isolator, PicoScope 6 crash

Postby PicoPilot » Mon Apr 25, 2016 3:32 pm

I'm aware of only one USB Isolator that supports USB 2.0 High Speed. It's made by Intona and it's not cheap. I don't have one myself.
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Re: USB isolator, PicoScope 6 crash

Postby Misan » Mon May 02, 2016 2:25 pm

@ PicoPilot:
Thanks for the Intona hint, I found that, too.

However, I’m afraid it won’t work with my PicoScope (2205) because of the limited device supply (300mA “nominal”), there is no option for an external supply, that’s bad.
It seems the scope is really power hungry, esp. when switching range, it doesn’t reliably work with a simple 500mA - rated wall plug supply or a crappy USB cable (unbelievable, but it is true ! ) from the isolator down to the scope.
Additionally the Intona price tag was a bit heavy (€ 250 +), compared to a versatile (hobby) PicoScope, so I went for the Olimex isolator.

Due to the clever design of the PicoScope the Full Speed USB connection isn’t really a problem in my non-professional application (if in any ?).
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Re: USB isolator, PicoScope 6 crash

Postby PicoPilot » Tue May 03, 2016 1:51 am

Hi Misan, I didn't notice the power limitation on the Intona USB Isolator until you mentioned it. You're right ... that's a showstopper and it's too expensive for what it does anyway.

I have the Picoscope 2205, as well. I didn't realize it would work on USB full speed. Now that you've indicated it does I'm thinking of trying the Olimex isolator. But my concern is not ground loop risk (I use a laptop running on battery which makes it floating). Rather, I hope the isolator will reduce the internal noise of my PC that appears as ripple in the signal displayed on the scope. But I'm not certain if the isolator will help with that.

I have a couple questions I hope you can respond to:

- In your previous post, you mentioned you connected an external power supply of 12V to the Picoscope while connected to the Olimex. My understanding is the Picoscope operates at 5V, the voltage standard for USB. What was the result of connecting 12V?

- You also indicated the Picoscope does not always receive sufficient current from the USB port (max 500ma). Have you measured the current draw of the Picoscope under different scenarios to determine the range of current required? What power supply are you successfully using with the Olimex?

Thanks in advance!
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Re: USB isolator, PicoScope 6 crash

Postby Misan » Tue May 03, 2016 9:42 pm

Hi PicoPilot,

No, I didn’t supply the PicoScope by 12V!
The PicoScope 5v supply is always taken from the USB connection (cable) and must be provided by the upstream USB port, called “host” (versus the PicoScope being called the “device”). USB data and power (supply for the PicoScope) must have identical (= connected) GND, otherwise host or device (or both) will be damaged.
Power direction must be from host to device, never try to supply from device to host (some active USB hubs will do and can damage the host / laptop when the host is switched off but the active hub isn’t).

So the isolator has to supply the PicoScope (and to provide the isolated data lines).

From the laptop the isolator may take up to 500mA (official USB 2.0 limit) and can provide isolated supply for the device, but only up to 350mA because it needs roughly 150mA itself.
If the device tries to take more (also in spikes), either the laptop or the isolator cut off and retry in milliseconds (data connection crashes).
However, my 2205 doesn’t even successfully start with the remaining 350mA from a laptop / PC.

But the Olimex isolator can be externally powered (DC 8 to 15V), too.
This supply is allowed to feed up to 750mA but is not galvanically isolated from the downstream USB device (the PicoScope), only from upstream laptop. The Olimex will combine USB and external power to max. 1A supply for the downstream USB device.

When I try a cheap wall plug supply (rated 9V, 500mA, but only 8.4V at 400mA) the PicoScope sometimes crashes, too.
I didn’t measure the current / voltage, there may be spikes / transients to be the cause (data line or power supply), I don’t know, but it doesn’t matter, it simply doesn’t work reliably.
Using a cheap 12V, 750mA wall plug supply (and the blue PicoScope USB cable) there are no such problems.

But to avoid the noise from the external power supply (plus the AC coupling to mains, which has a lot of spikes here as well) I use a 12V lead battery from solar panels (and a 1A fuse, just in case ! ) to supply the Olimex isolator.

Be aware of a “floating” laptop galvanically connected to the scope, due to stray capacitance it may float up to several volts, touching your device under test may stress or even damage it, depending how sensitive it is.

Always use a 1-5M resistor to define GND potential for your floating scope system, before connecting your probes and during test.
If applicable (depending on the test object) I (additionally) use a 1k resistor in series with two antiparallel 3mm LEDs from the scope’s BNC to (e.g. mains) GND to detect unexpected powerful voltage during test.

But Picotech may have special advice for using their USB scopes together with an isolated USB system?

Regards,
Misan
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Re: USB isolator, PicoScope 6 crash

Postby PicoPilot » Tue May 03, 2016 10:57 pm

Misan, I'm glad to connect with another 2205 user.

I think I now understand your previous reference to 12V was the power input to the Olimex isolator. It has its own internal step-down converter that regulates output to 5V for the Picoscope (or other USB devices).

I guess what's important is that your power supply is capable of providing sufficient current so the load from Picoscope is fully satisfied. I checked the current draw on my 2205 and it was 340mA, but I suppose it can vary a little depending on how many channels are in use, etc.

I am curious to know what is special about the blue Picoscope USB cable? Is it just a heavier gauge cable with lower voltage drop, or something else?
Last edited by PicoPilot on Wed May 04, 2016 6:17 am, edited 1 time in total.
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Re: USB isolator, PicoScope 6 crash

Postby Martyn » Wed May 04, 2016 6:09 am

Yes we use heavier gauge cable to reduce voltage drop.
We also connect the earth pin and shield at the PC end, something that should be done inside the PC, however we have found from experience this is not always done well.
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Re: USB isolator, PicoScope 6 crash

Postby bennog » Thu May 05, 2016 1:27 pm

Got mail from Franell today with a full-speed USB2 isolator capable of doing 500mA

http://www.farnell.com/datasheets/1929481.pdf

about 30 euro so it is possible worth trying out.

P.S. did some more reading of the PDF. It is without ESD protection chips and no USB connectors.
At the bottom is the schematic how to connect it to normal USB connectors.
So you need a little soldering skills to let this device work.

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Re: USB isolator, PicoScope 6 crash

Postby PicoPilot » Thu May 05, 2016 5:47 pm

I find it a little odd that this Murata isolator requires 5v power input and then uses an LDO to output ~5v power, essentially the same voltage level. They could have just implemented a pass-through for the external 5v power input to supply the device.

Usually a voltage converter is employed when you want want to provide the flexibility of a higher range of voltage input and then step down to 5v. Maybe it's used purely for noise reduction or line regulation?
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Re: USB isolator, PicoScope 6 crash

Postby bennog » Thu May 05, 2016 6:24 pm

They probably only do overcurrent protection with some power mosfet or something like that.

If I had more time I would buy one and build a fully working converter
And then the question will it work with the picoscope :?:

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Re: USB isolator, PicoScope 6 crash

Postby Misan » Fri May 06, 2016 2:25 pm

Re: Murata isolator module

@PicoPilot:
“... a passthrough for the external 5V power to supply the device ...”
Nah, the “external power” is on the upstream side and must be connected to host GND / reference, to supply the (active) pull ups for the data lines. So the voltage must be 5V (4.6 to 5.25).
To run the isolated downstream ports it also supplies the DC/DC converter for both USB outputs, drawing up to 1.3A from the primary source.
This concept may suggest to take that supply from a special powerful USB socket your laptop may have (called “charging downstream port”), but this would be a bad idea for several reasons.
However, do not connect the external +5V to the laptop’s USB +5V.
For the LDO: It has significant advantage in respect to emitted noise …

@Benno:
It is still just 2.0 Full Speed + a lot of work, why bother?
The Olimex USB-ISO is €29.95 (Olimex) or $39.00 (amazon), ready to go and it works, at least with my 2205 (and my iMac's USB port).
Yes, a High Speed module would be a nice project -
https://www.youtube.com/watch?v=f5tFunedEYM
8)
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