Hi Fazi,
As I understand it, you would like to use a DIO line to start your counter task. Start triggers for counter tasks must be set using an the arm start trigger.
Here is a great knowledgebase that talks about using arm start triggers on M Series devices such as the PCI 6221. In addition, I have taken the Meas Two Edge Separation-Buffered-Continuous.vi example (Help » Find Examples » Hardware Input and Output » DAQmx » Counter Measurements » Two-Signal Edge-Seperation) and added an arm start trigger below. You will need to connect the digital start trigger line to a PFI line to use it as the arm start trigger.

I hope this helps,
Paul C.
Message Edited by Paul C. on 10-23-2007 11:06 AM