It sounds like your sensor just puts out simple high/low pulses per revolution. If so, you could probably use one of the counters in a cog to detect the pulse transistions. What voltages are we talking about here 12v, 5v or 3.3v? You will probably need to use a resistor or a transistor or level shifter to condition the incoming signal to be appropriate for the input pin of the propeller board being used.
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It should, but if the RPM pulses go low twice per revolution then counting for 500mS should give you the RPM without any additional calculations.