Access to USB from inside the PC is done at the PC's rate, and not at the rate needed for the data sampling. If you use that USB/FIFO module, it allows a few samples be buffered in the FIFO, but, ... and it's a critical "but," ... if you need to take more samples than what the FIFO will hold, there's no guarantee that the PC will grab the data in time to avoid overflowing the FIFO.

That's why I say one should use a local RAM buffer. You see, one has to be sampling before the trigger occurs if one intends to acquire the history of the "blessed event." Often, it's useful to have history of the events leading up to the trigger condition, as well as what happens immediately afterward. If you're sampling signal states, a few (< 32K) samples are usually adequate. If you're capturing serial data or looking at the data from an external source, it's often necessary to capture LOTS of it and use the computer to search it for sequences of data afterward.

If all you're going to do is look for the occurrence of a specific pattern and examine the 100 samples before and after, a pretty simple approach will work. Unfortunately, in order to have the history, you must sample continuously until you encounter your "trigger" event.

RE