User Guide

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