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PXI-6562 DDR generation and acquisition at the same time

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HI,

 

I have just started with PXI-6552 DDR mode.  My question is:

1. Can the PXI-6562 be set to using the on-board clock for generation and strobe for acquisition at the same time?

 

The on-clock clock rate would be the same as strobe rate. However, these 2 clock sources are not synchronized.

We will use the PFI to trigger the acquisition somehow.  I saw an example code for doing that but it is not in DDR mode. Just want to make sure it can do that.

 

2. I noticed that there could be a bandwidth constraint for Fetching the acquired data even with PXI. What is the max bandwidth the PXI can transfer data to to PC?  I want to fetch the data sample by sample at DDR rate 200MHz. That is data rate of 400MB per second. Is that possible? If not am I going to store all the data in the board memory and fetch them in one shot?

 

Thanks!

 

Andy

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Accepted by topic author kkwong

Hey Andy,

 

Check out the HSDIO Help documentation, specifically on the Data Rate Multiplier topic. You can also check out the other information on DDR, such as Data Position, Generation Considerations, and Acquisition Considerations when using DDR.

 

To answer your first question, yes, you can use the Onboard clock for Generation, and the Strobe for Acquisition in the DDR mode. In the help it states that: "For generation sessions, the device generates data on the lower eight channels of the NI 656x (DIO <0..7>), and for acquisition sessions the device acquires data on the upper eight channels (DIO <8..15>)." As for the example that you saw, I'm assuming you are talking about the Gen and Acq, Source Synchronous example, and yes, you can set up this example to use DDR mode, as long as you change the channels to abide by the statement above.

 

Now for your 2nd question, regarding streaming of your data across the PXI bus. The PXI bus is basically the same as the PCI bus in that you can realistically get up to 110 MB/sec of streaming bandwidth, but depending on what other data going across the bus (since it is a shared bus), you might only get around 90-100 MB/sec of bandwidth. Other considerations are whether or not you are using MXI to connect to a PC, but this should only have a slight effect on the streaming bandwidth. So at 200 MHz DDR, you will probably not be able to stream that much data continuously for a long period of time before you overflow. Now you do get twice the amount of memory per channel when you use DDR ("Memory Usage—Memory size in samples is effectively doubled since the data width and channel count are halved."), but depending on what memory option of your 6562 you have, you will only be allowed to store a certain amount of waveforms onto your onboard memory.

 

I hope this helps. Please let us know if you have any further questions or concerns. Thanks, and have a great day.

 

Regards,

DJ L.

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Thanks DJ,

 

Your information is very helpful. My board should have 128Mb per channel memory. So I can store up to 256Mb/channel for DDR.  Then my next question is when I fetch these 256MB (since I got 8 channels for acquisition) data, do we need to fetch them in one time?  I think it needs to fetch couple times as 256MB needs lot of memory to store. I prefer to fetching and saving  the data with a loop.   Thanks!

 

Andy 

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Hey Andy,

 

Depending on how much data you are acquiring, you could go either way. If you are wanting to stream data to memory, and then to disk, while you are acquiring data, you can do this by using Streaming. First, check out this document called High-Speed Data Streaming: Programming and Benchmarks. After understanding how you will have to stream to memory, and then from memory to disk, check out the Example programs linked at the bottom for NI-HSDIO Stream to Disk Examples. Hope this helps. Please let us know if you have further questions or concerns. Thanks, and have a great day.

 

Regards,

DJ L.

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