National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 73)

© National Instruments Corporation 73 NI cDAQ-9172 User Guide and Specifications
Even if the Source pulses are long, the counter increments only once for
each Source pulse.
Normally, the counter value and Counter n Internal Output signals change
synchronously to the Source signal. With duplicate count prevention, the
counter value and Counter n Internal Output signals change synchronously
to the 80 MHz Timebase.
Note 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
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 x) 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 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. The NI cDAQ-9172 chassis synchronizes these signals
before presenting them to the internal counter.
The NI cDAQ-9172 chassis uses one of three synchronization methods:
• 80 MHz source mode
• Other internal source mode
• External source mode
In NI-DAQmx, the chassis uses 80 MHz source mode if you perform the
following:
• Perform a position measurement
• Select duplicate count prevention