What is the maximum number of bytes that may be sent at one time using the USB-8451 SPI interface? I understand that the data is sent in 8-bit chunks with a 10-20 usec delay between bytes but are the number of bytes limited to an on-board register or can this rate be sustained indefinitely?
You are correct, there is a delay of 0.08333μs to 20.83μs between every 8 bits as described on page 7-2 of the NI-845x Software User Manual. This rate is not sustained indefinately. There will also be a delay of 1 and 2 ms to refill the buffer every 100-bytes.
I actually answered a similar question in more detail not too long ago on this post.
Hope this helps, let me know if you have further questions.