Hello
What Stuart says is correct, I've attached a link to a knowledge base that describes how to do this in traditional DAQ, this also provides a link to a knowledge base describing how to do this with DAQmx.
http://digital.ni.com/public.nsf/websearch/6CBFC550085B32EC8625699A0078DAD8?OpenDocument
Hope this helps
Hannah
Applications Engineer
National Instruments