Hi Mike,
Here is my serial interrupt handler using Darrel's Instant Interrupts;

SerialInt:
Hserin [STR buffer\45\$8e] 'read up to 45 bytes into array, end if value is hex 8e.
@ Int_Return

That's all there is to it. If you do a search for frequency measurement, you will find some code that Bruce put together using Instant Interrupts to measure pulse width, and I think you could make it work for you.

Good Luck,

Jerry