Hi Om -
The "may" part refers to all the system dependencies and limitations that accompany streaming in a computer. When you fetch data from the 5620 digitizer, that data is DMA'd across the PCI bus to RAM. This process is limited to the PCI bus's 133 MB/s bandwidth, which is closer to 100 MB/s in reality. Once this data gets there, the CPU has to scale it and form it into the datatype you want out. This takes time and a few memory transfers, as well.
If you're streaming to disk, you then have to write the data from RAM to your hard drive. Hard drives are
incredibly slow compared to the other parts of a PC, and you may only be able to sustain 20-25 MB/s. This can be increased by using new technologies like SATA2, a RAID array, or drives with fast spindle speeds (like 10k rev/min).
And all of that assumes you won't need to process the data as it's fetched, which further reduces your speed.
Attached are two simple benchmark programs that you can run to find your system's streaming rate. One of them streams to RAM, and the other to disk. The disk example returns unscaled raw data, so it should represent the fastest possible streaming rate to disk. The RAM example scales the data first. You will probably not be able to sustain a 1.25 MHz bandwidth when streaming to disk, though a newer controller is likely to be okay if streaming to RAM.
David Staab, CLA
Staff Systems Engineer
National Instruments