NI cDAQ-9178 - Other Counter Features

NI cDAQ-9178/9174 User Guide and Specifications 64 ni.com
generations, the counter stops generating pulses while the external trigger signal is low and resumes
when the signal goes high or vice versa.
When using a pause trigger, the pause trigger source is routed to the Counter n Gate signal input of
the counter.
Other Counter Features
The following sections list the other counter features available on the NI cDAQ-9178/9174.
Cascading Counters
You can internally route the Counter n Internal Output and Counter n TC signals of each counter to the
Gate inputs of the other counter. By cascading two counters together, you can effectively create a 64-bit
counter. By cascading counters, you also can enable other applications. For example, to improve the
accuracy of frequency measurements, use reciprocal frequency measurement, as described in the Large
Range of Frequencies with Two Counters section.
Prescaling
Prescaling allows the counter to count a signal that is faster than the maximum timebase of the counter.
The NI cDAQ-9178/9174 offers 8X and 2X prescaling on each counter (prescaling can be disabled).
Each prescaler consists of a small, simple counter that counts to eight (or two) and rolls over. This
counter can run faster than the larger counters, which simply count the rollovers of this smaller counter.
Thus, the prescaler acts as a frequency divider on the Source and puts out a frequency that is one-eighth
(or one-half) of what it is accepting as shown in Figure 56.
Figure 56. Prescaling
Prescaling is intended to be used for frequency measurement where the measurement is made on a
continuous, repetitive signal. The prescaling counter cannot be read; therefore, you cannot determine
how many edges have occurred since the previous rollover. Prescaling can be used for event counting
provided it is acceptable to have an error of up to seven (or one) ticks. Prescaling can be used when the
counter Source is an external signal. Prescaling is not available if the counter Source is one of the
internal timebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
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 the NI cDAQ-9178/9174 synchronizes these
signals before presenting them to the internal counter.
Depending on how you configure your device, the NI cDAQ-9178/9174 uses one of two synchronization
methods:
• 80 MHz Source Mode
• External or Internal Source Less than 20 MHz
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
01