measurement digial input or analog input

Hello,

I am trying to measure frequency using NI DAQmx task and then convert this to an RPM if at all possible.

I have the following hardware .

I have NI DAQ-9174 with the DI (NI 9421) modules  (8 channels) 

I have wired the proximity sensor and tachometer to ai0 and ai1 respectively which is PFI 1 and PFI 5  among these 8 channels in order to measure (pulse ,frequency , or periods) .  

What I am trying to do is to somehow convert this digital measurement into a RPM by using the DI module.

If not possible using the DI module, can I use AI module (NI 9229) in order to convert this analog measurement into a RPM.

Any help would be appreciated.

Mola

Test Engineer