Solution
According to the USB-6421 Theory of Operation, “Flexible routing allows you to route any counter to any of the digital I/O lines,” suggesting that any PFI pin can be used as a source, gate, output, etc. Default pins for counters exist for mioDAQ devices, and the pins are consistent across all models due to their identical counter capabilities. However, NI is not restricted to these pins and can use different PFI pins if desired.
In conclusion, the USB-6421 allows for flexible routing, meaning any PFI pin can be used as a counter (e.g., source, gate, output, etc.). This flexibility ensures that signals can be routed to counters as needed. Additionally, default pins for counters do exist for mioDAQ devices, providing a consistent pinout across all models due to their identical counter capabilities. Therefore, both flexible routing and default pin configurations are feasible options.