Reducing Sample period of digital input of NI-USB-6009

Hi,

I need to read a digital input line in the NI USB-6009 using NI Signal Express 2013. I have seleted 1 sample (on demand) as the Aquisition mode. I need to set a smaller sample period like 1 ms. But this gives the error sample period too short: "The current sample period is too short.  Please specify a longer sample period.".
I cannot understand the reason for this and a way to slove this.

Any help would be greatly appreciated!!

Thanks!!