I am trying to use the 6251 counters within a sweep function. It appears only one count can be used. Am I correct? If I am able to get both count 0 and count1 in the sweep menu only one shows up with number of samples to select. If I try to run a sweep function with two counters in the loop I get error messages that the second device is not available.
Is this a limitation of SE or the 6251 USB or my understanding of SE?
Your answer lies in the NI-DAQmx Help. The counters on your DAQ device are paired, and certain operations require a pair of counters instead of just one. When you generate a finite number of samples, the driver must use a pair of counters, so if you select Ctr0 it'll automatically grab Ctr1 along with it. When you generate one sample or continuous samples, only one counter is needed.
This is a hardware design limitation that can't be overcome, except by installing a second DAQ device or a device with several counters (like the 6602, 6608, or 6624).
David Staab, CLA Staff Systems Engineer National Instruments