Hello Jujian, Thank you for contacting National Instruments.
The amount of your back log is system dependant. It depends on the computer and the applications running on that computer. To set your buffer size, simply look at your maximum scan backlog and add 100 to prevent a buffer overflow error. In your case, this would be 1000 + 100 = 1100. I hope this helps, Sean C. Applications Engineer National Instruments
