Sorry, another queston, roll mode this time...

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Basil
Posts: 7
Joined: 03 Dec 2011 19:18

Sorry, another queston, roll mode this time...

Postby Basil » 04 Jan 2012 00:25

Happy New Year! Having re read several times, and hopefully assimilated Neil's excellent reply in my overload protection thread, in this section, I have now got another question. In automotive diagnostics we are often measuring what I call very slow waveforms, often in the sub 800 Hz region, sometimes down to 8Hz. Viewing such slow waveforms and seeing any glitches is proving tricky, so a while ago I bought the Roll Mode upgrade from you. I have been paying with it using an old signal generator I have, giving a square wave signal at 2 to 600 Hz, and capturing various sets of data at different speeds via the roll mode. By means of roll mode it is much easier to see all of the waveform over extended periods. But what effect does running out of internal memory buffer at these low frequencies have on the stored signal? When the red line has crossed the screen I believe this shows the buffer is full? After that, what is occurring? is the buffer plain full, and stays full, and the rest of the data is caught in real time on the PC? Or is the buffer emptied very fast and it refills? I assume the former as no glitch is evident in the traces. What I am getting at is when the buffer is filled, at these low frequencies will roll mode continue to store accurate traces and be reliable for seeing glitches from missed or corrupted injection or spark events, for example? To what sort of frequencies will roll mode store useful data once the buffer is filled? Apologies if the questions are naive, but the more I learn unfortunately the more questions I seem to have. Thanks.

Dataman (Neil Parker)
Posts: 810
Joined: 10 Nov 2011 09:51

Re: Sorry, another queston, roll mode this time...

Postby Dataman (Neil Parker) » 10 Jan 2012 09:01

Hello Basil,

The vertical red line indicates, that at least one sample of the waveform missing. The oscilloscope stores the samples to its internal buffer and the computer reads and processes them in real time (the internal buffer is a "dual port" memory and the computer can work in parallel with the scope). The problem occurs when a computer or its operating system is busy for a period of time longer than the time it takes for the scope to fill the entire memory. In this case the samples which cannot be written to the buffer are lost.

We have tested the maximum 5MS/s sampling rate on a fast computer and disc drive with minimum OS overhead and with the waveform display switched off, and it has worked. But in a "real computer" with a standard OS, the 5MS/s sampling rate usually does not work. The 2 MS/s sampling rate usually works and the 1 MS/s sampling rate works with no problem at all. If this is not the case then there are most likely some hardware problems (USB1.1 interface, or a very fragmented disc drive for example), or too many applications running on the computer.

I hope this helps.

Regards
Neil Parker
Dataman Programmers

Basil
Posts: 7
Joined: 03 Dec 2011 19:18

Re: Sorry, another queston, roll mode this time...

Postby Basil » 11 Jan 2012 10:58

Thanks again Neil, another detailed and clear reply, great support, and much appreciated.


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