Chapter 7 CountersNI 6232/6233 User Manual 7-38 ni.comNormally, the counter value and Counter n Internal Output signals changesynchronously to the Source signal. With duplicate count prevention, thecounter value and Counter n Internal Output signals change synchronouslyto the 80 MHz Timebase.Note that duplicate count prevention should only be used if the frequencyof the Source signal is 20 MHz or less.When To Use Duplicate Count PreventionYou should use duplicate count prevention if the following conditions aretrue.• You are making a counter measurement• You are using an external signal (such as PFI <0..5>) as the counterSource• The frequency of the external source is 20 MHz or less• You can have the counter value and output to change synchronouslywith the 80 MHz TimebaseIn all other cases, you should not use duplicate count prevention.Enabling Duplicate Count Prevention in NI-DAQmxYou can enable duplicate count prevention in NI-DAQmx by setting theEnable Duplicate Count Prevention attribute/property. For specificinformation on finding the Enable Duplicate Count Preventionattribute/property, refer to the help file for the API you are using.Synchronization ModesThe 32-bit counter counts up or down synchronously with the Sourcesignal. The Gate signal and other counter inputs are asynchronous to theSource signal, so M Series devices synchronize these signals beforepresenting them to the internal counter.M Series devices use one of three synchronization methods:• 80 MHz source mode• Other internal source mode• External source mode
