I am using a cDAQ-9172 and I have an NI 9401 module to achieve the two-signal edge-separation measurement. Is there a way to substitute an internal signal to the source pin? In my observation, i find it inefficient to use 3 pins to measure time for two signals. Just a thought, no offense meant.
A two-edge separation measurement will require two input signals for the counter. They should be connected to CTR 0 GATE and CTR 0 AUX (Pins 16 and 17). These pinouts and the required terminals can be found in the Device Terminals Help section of the NI-DAQmx Help or in the NI cDAQ-9172 User Guide. By default, the internal timebase is routed to the source pin so you do not have to connect an external signal. If you wanted to route another signal to the source, you can use property nodes in LabVIEW as seen below or the DAQmxSetCICtrTimebaseSrc() and DAQmxSetCICtrTimebaseRate() functions. For more information on setting these properties, you can look at this article which sets similar properties.
You will just have to use the two functions I mentioned in the last post before you start your task. If you need additional information on those functions, the NI-DAQmx C Reference Help is useful. It can be found under Start » Programs » National Instruments » NI-DAQ » NI-DAQmx C Reference Help if you are using Windows.