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Chapter 7 Counters
NI 6238/6239 User Manual 7-38 ni.com
counter value and Counter n Internal Output signals change synchronously
to the 80 MHz Timebase.
Note that duplicate count prevention should only be used if the frequency
of the Source signal is 20 MHz or less.
When To Use Duplicate Count Prevention
You should use duplicate count prevention if the following conditions are
true.
• You are making a counter measurement
• You are using an external signal (such as PFI <0..5>) as the counter
Source
• The frequency of the external source is 20 MHz or less
• You can have the counter value and output to change synchronously
with the 80 MHz Timebase
In all other cases, you should not use duplicate count prevention.
Enabling Duplicate Count Prevention in NI-DAQmx
You can enable duplicate count prevention in NI-DAQmx by setting the
Enable Duplicate Count Prevention attribute/property. For specific
information on finding the Enable Duplicate Count Prevention
attribute/property, refer to the help file for the API you are using.
Synchronization Modes
The 32-bit counter counts up or down synchronously with the Source
signal. The Gate signal and other counter inputs are asynchronous to the
Source signal, so M Series devices synchronize these signals before
presenting 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
In DAQmx, the device uses 80 MHz source mode if the user performs the
following:
• Performs a position measurement
• Selects duplicate count prevention