6-2 | ni.com
Chapter 6 Counters
Counter Timing Engine
Unlike analog input, analog output, digital input, and digital output, the cDAQ chassis counters
do not have the ability to divide down a timebase to produce an internal counter sample clock.
For sample clocked operations, an external signal must be provided to supply a clock source. The
source can be any of the following signals:
• AI Sample Clock
• AI Start Trigger
• AI Reference Trigger
• AO Sample Clock
• DI Sample Clock
• DI Start Trigger
• DO Sample Clock
•CTR n Internal Output
• Freq Out
• PFI
• Change Detection Event
• Analog Comparison Event
Not all timed counter operations require a sample clock. For example, a simple buffered pulse
width measurement latches in data on each edge of a pulse. For this measurement, the measured
signal determines when data is latched in. These operations are referred to as implicit timed
operations. However, many of the same measurements can be clocked at an interval with a
sample clock. These are referred to as sample clocked operations. Table 6-1 shows the different
options for the different measurements.
Table 6-1. Counter Timing Measurements
Measurement
Implicit
Timing Support
Sample Clocked
Timing Support
Buffered Edge Count No Yes
Buffered Pulse Width Yes Yes
Buffered Pulse Yes Yes
Buffered Semi-Period Yes No
Buffered Frequency Yes Yes
Buffered Period Yes Yes
Buffered Position No Yes
Buffered Two-Signal Edge Separation Yes Yes
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