655x stopping strobe clock

[mlloyd]

To those familiar with the ways of the 655x...
I have a finite length acquisition that will be performed with the PCI-6551. If the external strobe clock input (via the DDC connector) stops oscillating after the acquisition has stopped, will this cause problems in my LabVIEW program? What if the external strobe signal stops oscillating before the acquisition has completed?

Thanks for your help in this matter!
Regards,
mlloyd

[Alan_L]

mlloyd,

Here are answers to your two questions:
1) If your external clock stops pulsing AFTER the 655x finishes acquiring its data samples, then there will be absolultely no effect on your application.
2) However, if the clock stops BEFORE all of the requested points are buffered, your HSDIO Read VI will eventually time out and return an error indication (depending on how much time you wired into the timeout terminal). Wiring a -1 to the timeout terminal indicates that you want the VI to wait forever.

Best Regards,
Alan L
Applications Engineer

[mlloyd]

That's good news -- THANKS! There is hope that my application will indeed run...

Regards,
mlloyd

[Allen H]

Hi mlloyd,

To add to earlier comments...
On SMC-based digital boards (655x, 654x, 656x), in general a continuous and free-running clock is required at all times between the initiate function and the end of the acquisition or generation. The clocking and timing circuitry must remain locked to the clock in order to function properly.

If the sample clock is removed before the acquisition has completed, the board will return a hardware clocking error.

However, there may still be a way to do what you need. Do you need to pause the acquisition for some amount of time?

Allen

[mlloyd]

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@Allen H. wrote:
Do you need to pause the acquisition for some amount of time?

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It's not that I need to pause the acquisition but that I need to turn off all clocks when not generating or acquiring data. Our board is very sensitive to any noise and so we want to keep things as quiet as possible.

Knowing that I can turn off the clock after acquisition completes, I should be okay. I think this will accomplish what we are trying to do!

Do you know how long it takes for "acquisition to complete"? i.e. how long will the clock have to keep running after the last bit of data of a finite data acquisition is received?

[Alan_L]

mlloyd,

You can stop the clock immediately after the last datapoint is returned. However, remember as Allen mentioned, the 655x needs a free running clock for its PLL. Once you stop your external strobe, the board will lose lock and return an error. To begin another acquisition or generation you'll need to restart the clock and reset the device.

I hope this helps!

-Alan

[mlloyd]

Got it! I'll keep all this in mind when structuring my VI, and hopefully the integration will go smoothly!