Propscope Buffer Size & USB Safety Questions

SystemTheory

I'm an engineer·looking to use the Propscope (or similar tool) to teach data capture for·online education materials. I find no information on the Propscope specification of the USB isolation or wiring safery procedures.

I read·a post on another support group where a person fried his (non-propscope!) USB oscilloscope equipment by connecting probes the wrong way·while using a non-isolated USB device.

I also need to consider the storage buffer size. For example at a 1ms sample rate for two channels, how many seconds of data can the Propscope capture? Once captured, what is the data structure and resolution upon saving the file to disk?

Post Edited (SystemTheory) : 1/4/2010 8:53:30 PM GMT

David Carrier

SystemTheory,

The PropScope's high-voltage protection is on the input side, not the USB side. It does share the ground with the PC that it is connected to, through the USB port. The PropScope's inputs can be directly connected to +/- 200 VDC, relative to ground, without damaging the 'scope or the computer it is connected to. It won't be able to measure anything outside of +/- 10 VDC, but it won't incur damage either. 200 VDC is a conservative number, so you would have to apply significantly more voltage to guarantee damage to the PropScope. When the PropScope is connected to a voltage high enough to cause damage, the resistors, capacitors, and traces in the input dividers generally blow open, preventing current from flowing into the PropScope. The voltages needed to cause enough damage to the PropScope to force current into the USB port would be unreasonably high. We don't have any official numbers, but I would estimate that it would take thousands, if not tens of thousands, of volts. If you use 10x probes, they provide significantly more protection, and any voltage high enough to damage the PropScope would have already damaged the probe.

As for the PropScope software, there a stream function under the file menu. It lets you output data as plain text (.txt), comma separated values (.csv), or Matlab MAT-file (.mat), as well as a screen shot (.jpg). I just tried it out using .csv a file at a 1ms sample rate, and it used a buffer size of 226 samples per channel. The PropScope software was written by Hanno Sanders, the author of ViewPort. Hanno actively uses these forums, so if you have comments or requests, feel free to post them, and He will do what he can.

- David Carrier

Hanno

Thanks David- great explanation (as usual). My wife already complains about my ego, so no need to capitalize "He" when referring to me [noparse]:)[/noparse] But seriously- please make use of these forums. I'll help as quickly as I can. I would love to see some success stories too!

What have YOU done with your PropScope today?
Hanno

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Co-author of the official Propeller Guide- available at Amazon
Developer of ViewPort, the premier visual debugger for the Propeller (read the review here, thread here),
12Blocks, the block-based programming environment (thread here)
and PropScope, the multi-function USB oscilloscope/function generator/logic analyzer

tdlivings

David
·I think a point that really needs to be stressed is that the ground is not isolated.
As you say

· The PropScope's high-voltage protection is on the input side, not the USB side. It does share the ground with the PC that it is connected to, through the USB port

When you are connected to a laptop you will be connecting the laptop ground to what ever potential you connect the ground
clip to, and the laptop mother board might not like -15 volts or it might take out your -15 supply.

We do not always connect to where we think we are connecting, how many of us have thought a test post on a circuit board was
ground and it turned out to be -15.

The real safety issue is if someone tries to measure the AC coming out of the wall to see how clean it is without using an isolation transformer and
upon connection the ground clip will get a blue flash.

It is not a propscope only issue all non isolated usb instruments have the problem. They put the laptop or desktop in harms
way with the ground clip.

Tom
·

SystemTheory

tdlivings said...
David
·I think a point that really needs to be stressed is that the ground is not isolated ...

It is not a propscope only issue all non isolated usb instruments have the problem. They put the laptop or desktop in harms
way with the ground clip.

Tom

Tom, thanks for the description of the safety concern. This is roughly what I've read occured with another non-isolated USB scope, something on the ground clip that harmed the scope but in that case did not harm the USB port. I know isolation would be more costly so Propscope offers value.

David, thanks for doing the·1ms two channel sample test.·I still have this bandwidth question for anyone with knowledge.

So·the total sample time·in a log file is limited to the buffer length,·e.g., 226 milliseconds·total sample time at 1ms rate·capturing two channels? I am hoping to capture the response of a mechanical source or damping element·over several seconds or more to teach modeling of sources and sinks. I could relax the sample rate to 0.01 second but it would be nice to track a better resolution. Thanks for the answers.