PXIe-7820 DRAM needed or not?

[yejus]

Hello,

Noob here. I am looking to buy a PXIe-782x Digital Reconfigurable I/O Module (PXI Digital Reconfigurable I/O Module - NI) which comes in two kinds -- with DRAM included or not. However, I am having a little trouble understanding what exactly the DRAM does and if I require it. All I need for my purpose is to configure the module in output mode from LabVIEW and have the 128 channels of the module be in different states (on or off) for, say, 1024 timestamps. For example, at t = 0, I want channel 1 to be OFF, channel 2 to be ON, etc.; at t = 0.1 ms, I want channel 1 to be ON, channel 2 to be OFF, etc.; and 1022 other timesteps.

Could someone please explain to me if I need to buy the DRAM version of the module, considering the number of lines I need to send to the module for my application? 

[Terry_ALE]

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@yejus wrote:

Hello,

Noob here. I am looking to buy a PXIe-782x Digital Reconfigurable I/O Module (PXI Digital Reconfigurable I/O Module - NI) which comes in two kinds -- with DRAM included or not. However, I am having a little trouble understanding what exactly the DRAM does and if I require it. All I need for my purpose is to configure the module in output mode from LabVIEW and have the 128 channels of the module be in different states (on or off) for, say, 1024 timestamps. For example, at t = 0, I want channel 1 to be OFF, channel 2 to be ON, etc.; at t = 0.1 ms, I want channel 1 to be ON, channel 2 to be OFF, etc.; and 1022 other timesteps.

Could someone please explain to me if I need to buy the DRAM version of the module, considering the number of lines I need to send to the module for my application? 

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Hard to tell from what is mentioned above.

The FPGA itself has storage (LUTs, BRAM, etc).  If that storage is not sufficient, then we use DRAM.  DRAM is off-chip, onboard memory.

DRAM adds complexity to an FPGA design so you do not want to use it if you don't have to (let alone pay for it when you do not need it).

[UMASO]

Hello, do you just want to have a high channel density digital output module?  Sounds like you would like to generate digital waveform from multiple channels.  In that case, PXIe-653x digital output module supports DAQmx driver API with which users can easily do programming on LabVIEW and other development environment.  PXIe-782x, on the other hand, it is not supported by DAQmx driver but it is a digital modules of high channel density.  If you are familiar with FPGA programming, it is relatively easy to implement a function of  digital waveform generator.  However, if you are not, it takes some time to accomplish such a functionality by yourself.  Unfortunately, I cannot find an example which has a good fit for your situation.  

As for necessity of DRAM, it is a memory placed right next to FPGA on PXI board.  It can store a relatively large amount of data such as 512MB and it can transfer data to/from FPGA faster than data transfer between PC and FPGA.  In your use case as a digital waveform generator, lets say you would like to generate a long waveform of 10M sample from all 128 channels on PXIe-782x, and sampling rate is 40MSps.  In that case, total data throughput from digital output is 128 channel * 40M [samples/sec] / 8 [channels / byte] = 640Mbyte / sec.  To continuously provide data to 128 digital output channel, there are two options.  Option 1 is that data is continuously transferred from PC to FPGA at 640MByte/sec.  Option 2 is to download the 10M sample long data for all the 128 channels on DRAM first, then, start generation and provide data from DRAM to digital output continuously.  Either way, you can accomplish digital waveform generation, but CPU is less busy in option 2 than in option 1, because DRAM and FPGA do the digital waveform generation, while CPU does nothing after it transfers waveform from PC to FPGA.  Usually, DRAM-FPGA data transfer rate is faster than that of between PC-FPGA for devices other than PXIe-782x devices.  But for PXIe-782x devices, DRAM is less useful than for the other FPGA devices.  So, in my opinion, you do not need to choose 7821/7822 which are with DRAMs.  

Anyways, I would recommend you to contact NI sales representative to choose a right device for your requirement.