Peter,
Based on your discussion about possible solutions, it seems that you wish to acquire data continuously each time you receive a trigger signal. In this case, yes, you do want to use scan clock gating or counters to create you own scan clock signal. Usually, for the rate at which you acquire data, there is not enough time to call the functions to set up the triggers, clear them, and then reconfigure them for the next round.
I recommend starting with the examples in the NI-DAQ >> Examples >> AI directory, such as the SCANsingleBufAsync to start with. The SCANsngleBufAsyncExtScan_ESeries shows how to apply an external clock signal. You would have to use one of the counters running the STCgenerateRepeatedTriggeredPulse example and wire that to
the pin expecting the external scan clock signal. Then, apply your trigger signal to the gate pin of the counter running the example. For a double buffered operation, search the http://www.ni.com/support pages for "double buffered scan", and you will see the "Double-Buffered Asynchronous Scanning in Microsoft Visual C++ with NI-DAQ" example. If you are not using Visual C, then you can still see the function calls in the C file of that example.
Also, the NI-DAQ User Manual for PC Compatibles is a very helpful reference when dealing with NI-DAQ function calls, because it describes how the functions fit together. The NI-DAQ Help file describes the details of each function, as well as the parameter details.
Regards,
Geneva L.
Applications Engineer
National Instruments
http://www.ni.com/support