Is it possible to use internal clock for buffered counter tasks?

Hi,

Hardware: USB 6251 Mass Term

Software: LabView 2011 SP1

I need to measure the angular position of a lever and the speed of a treadmill belt. I am using two quadrature encoders for this. To accurately calculate the velocity I would like to use buffered position measurements using one of the counters available on board. I understand that for this technique I need to provide a sample clock for the counter. I wonder whether it is possible to use the internal timebase for this. Note that both of my counters are used so I cannot generate a clock signal with them.

I found two contradictory pages relating to my problem:

1) http://digital.ni.com/public.nsf/allkb/EA7FFFEAFC3E1D85862572F700699530

2) http://digital.ni.com/public.nsf/allkb/775290A3121D1FFC862577140074D3B3

The first one says I can use the 100KHz internal clock, and the second one says I must have an external clock.

Any comments/solutions?

Javad