We have a common re-occuring problem reported with the RPM input.
The customer reports (from http://vems.hu/www.vems.co.uk/forum/index.php?topic=256 )
"I am feeding the RPM input with my function generator. Nothing else is connected. The dips come sporadic. Sometimes the RPM display is stable up to 20 seconds, sometimes they appear after 1 second. The input signal and the pullup resistor on pin 11 has no influence on the dips.
The dip shows a RPM which is about a factor of 2,5 lower (2205 instead of 5555rpm or 1151 instead of 2860 ...). I think the dip is not a problem of the input signal, it must be a firmware problem (missing int or something like this).
To understand more of the analog and rpm input, a schematic would really help. I haven't found one on the wiki.
I have the newest firmware (round_2007-04-25). I don't use signalgen.exe. I feed the rpm input with a square wave signal (0 to 12V/ duty cycle 50%) from an external generator."