multifetch

[Mark Mutton]

Hi, I am using a 5124 in conjunction with a 8196 controller. I have a good problem here in that when  using the ni example vi multi record fetch more than available memory (with a few modifications i.e. no display and the multifetch is not in any loop) , i am able to collect way more information than I should be able to. Let me be more spacific I am collecting an ultrasonic pulse every 200usec. The 5124 is receiving a trigger from the ultrasonic hardware and i am collecting 500 points of data within the 200usecs at 32bits(4 bytes).  The 5124 has 8M onboard memory so this says without fetching the data during equation I should only be able to collect about 4000 waveforms.  I have found that this upper limit is more like 6500. Why is this?  

Also using the fetch more than available option I am able to collect 180,000 waveforms without overwriting any data. How is this possible when i am not fetching any data during acquistion. The only thing i can think of is that the A/D 5124 is dumping some of this data into virtual memory. (win XP)

[Sean_C.]

Hello Mark,

How are you fetching 32-bit samples from the PXI-5124?  At most this device will transfer 16-bits for every sample.  Therefore, you should be able to acquire no more than 8,000 records before filling the 8 MB onboard memory.  (It is actually less because of additional overhead for multi-record acquisitions, though I would expect more than 6,500 records.)

If you attach your modified code, I would be happy to take a look.

Regards,

Sean Close

Product Support Engineer, High Speed Digitizers

[Mark Mutton]

Sorry about the 32 bit mix up, you are right it is transfering in 16-bit format. I guess all i really need to know is why when i enable the more records than memory property node of the ni scope am i am able to collect 180K 500 point waveforms. There has to be some communication between the computer and the digitizer during acquisition. Again my digitizer has only 8M onboard memory.

Mark

[dklipec]

Maybe I'm replying to this a little late, but:

When you "Enable Records > Memory," all you are doing is allowing the digitizer to overwrite completed records that are stored onboard.  If you were to wait until the acquisition is complete to fetch any records - you'd get an error indicating that the first N records were overwritten where N is (Num Records - Max Onboard Records), assuming Num Records > Max Onboard.

However, your program starts fetching immediately.  As the acquisition is in progress records that are complete will be transferred to host memory.  So even though you only have 8M of memory onboard, that is just a big FIFO for transferring to the host if you start fetching immediately.  As long as the host can transfer the records from the device before they are overwritten, you can acquire substantially more than the device itself can hold.