A typical place to get a tach signal is the negative side of a coil primary.
You will get one pulse per spark plug firing there, which allows you more
resolution with less processing time than you can get from a
once-per-rotation signal. (snappier hi-res result.) Obviously simple
division is then done in your code - this is why you see dip switches or
jumper settings on most commercial tachometers for 4, 6, or 8 cylinder
vehicles.
You can shape the pulse any way you like - my preferred method is a zener to
limit the amplitude and an optocoupler with a shmitt, schmidt, or is it
schmitt trigger (sp?) output side. You'll get a nice clean and safe signal
this way. Don't forget to put a cap across the output, between say .1 & 1
uF. Also, realize that at different RPM's the duty cycle will change enough
to foul up your reading if you can't catch short pulses at one end and long,
bareley separated ones at the other.

Chris

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