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Ist there a possibility for a acquisiton stop condition

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Ist there a possibility for a acquisiton stop condition

Postby AngusX » Fri Dec 11, 2009 1:39 pm

Hello,

i want to get frames from a RS485 bus with a 5203. While trying, i thougt it was handy to use the waveform buffers for the single frames.
I set up the trigger condition, so that it respond to the inter frame gap. Now i've got the beginning of the frame.
The problem is, that the scope samples as long as the time was set in PicoScope (1, 10, 50, 100 ms, and so on). But the frames have got legth between 2 an 200 ms. So i can only decide to get more than one frame or to loose the rest of it.

Is if there is a possibility, similar to the trigger, to stop the current data acquisition so that the next trigger event will catch the really next frame and put it in the next wafeform buffer.

Regards, Andreas.
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Re: Ist there a possibility for a acquisiton stop condition

Postby ziko » Fri Dec 11, 2009 2:47 pm

Well I suppose this depends on a number of factors:

What is the data rate of the signal?
How long is each event?
How many events do you want to capture?
What is the time between events?

Kind regards
Ziko

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Re: Ist there a possibility for a acquisiton stop condition

Postby AngusX » Fri Dec 11, 2009 3:05 pm

Hello ziko,

What is the data rate of the signal?

Actually 38400 bps.

How long is each event?

That depends on the definition of an event.
A bit has got the length of 25 ns, a byte has got 250 ns an a complete telegram varies between 2 and 200 ms.

How many events do you want to capture?

As much as possible. While PicoScope provides up to 1000 wafveform buffers also up to 1000 telegrams if possible.

What is the time between events?

The time between the telegrams is approx. 2 to 5 ms. I set the trigger to Interval trigger, rising edge 500 ns (according to the baud rate 2 bytes long) and it works very well (triggering on the start bit of the first byte of a frame). As stop condition i think level dropout will be sufficient.

Regards, Andreas.
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Re: Ist there a possibility for a acquisiton stop condition

Postby AngusX » Mon Dec 14, 2009 9:57 am

Hello,

maybe i can make it more clearly with an example.

Let's say i get a short request frame with a length of 5 ms. After the frame there is a 'gap' of 4 ms with no data.
Then the answer will be send, a telegram of 80 ms.

What i want is, that both frames will be captured in their own waveform buffer. But i'm not shure if this is possible with PicoScope or at all, because the scope must stop acquiring the current frame at the stop condition and rearm it's trigger for the next frame.
I think the problem could be, that the trigger hasn't be rearmed at fixed intervals but after a stop condition which appears after an indefinite time.

Regards, Andreas.
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Re: Ist there a possibility for a acquisiton stop condition

Postby ziko » Tue Dec 15, 2009 12:20 pm

Hi well there is a mode called rapid block mode on the 5000 where it captures to the memory of the device and does not transfer it to the PC instead it moves to the next segment of memory. The time between trigger re-arming reduces to 600us. However this can only be done for a finite time really. At the rates you will be sampling at it is not achievable.

When you say 38400 bps I assume you mean the baud rate rather than the bit rate?

So if we were to look at 25ns as the pulse width you would need to sample at the very least three samples across this pulse. Which would equate to the nearest sample rate of 125MS/s, perhaps sampling at 250MS/s is better. So if you were to sample at that rate you will get half a second worth of data on the 5204.

For the trigger to re-arm again it will take a couple of hundred milliseconds or more if you are transferring data to your PC.

Hope this helps.
Ziko

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