Using counter in NI-6220

Hello:

I'm using NI PCIe-6220 DAQ on Ubuntu 17.20 with comedi driver. I'm trying to set it up to read one line TTL signal from an RPM sensor (ROS). Thus I have two questions:

1. Does anyone have an example C code for this device using comedi driver?
2. The specification for 6220 indicates that there are 6 inputs for a counter: A, Z, B, SRC, GATE, & AUX. What is the difference between them? If I just have one signal going up and down, where should it be connected?

Thank you.