Hi earny,
Counting the 80 MHz timebase takes about 53.69 seconds to roll over--the rollover is the usual reason to want to use a lower frequency timebase and doesn't really apply to your situation. I don't believe that the timebase is the cause of your problem. However, to answer your question, you may set the counter timebase with a DAQmx Property. In VB.NET you use CounterTimebaseSource in the CIChannel class.
What I think is more likely happening is that you are seeing a small amount of noise or glitching on your line, which is resulting in the card reading duplicate edges on some transitions. This would manifest itself as reading in an occasional high frequency (e.g. 4000 ticks of the 80 MHz timebase is a 50 microsecond period).
I suggest to try using digital filtering (see M Series User Manual). With digital filtering enabled, the counter would only register pulses larger than {125 ns, 6.425 us, or 2.56 ms). If your max frequency is only 300 Hz, you should be able to safely set the 2.56 ms digital filter, which is guaranteed to pass through any signal up to ~390 Hz. Any high frequency glitches that occur during the transition of your signal would be filtered out.
To enable digital filtering and set the minimum pulse width, use the CIChannel.FrequencyDigitalFilterEnable and CIChannel.FrequencyDigitalFilterMinimumPulseWidth properties.
I hope this helps, let me know how it goes!
Best Regards,
John Passiak
