User Guide

NI cDAQ-9172 User Guide and Specifications 44 ni.com

Controlling the Direction of Counting

In edge counting applications, the counter can count up or down. You can

configure the counter to do the following:

• Always count up

• Always count down

• Count up when the Counter n B input is high; count down when

it is low

For information on connecting counter signals, refer to the Default

Counter/Timer Routing section.

Pulse-Width Measurement

In pulse-width measurements, the counter measures the width of a pulse on

its Gate input signal. You can configure the counter to measure the width

of high pulses or low pulses on the Gate signal.

You can route an internal or external periodic clock signal (with a known

period) to the Source input of the counter. The counter counts the number

of rising (or falling) edges on the Source signal while the pulse on the Gate

signal is active.

You can calculate the pulse width by multiplying the period of the Source

signal by the number of edges returned by the counter.

A pulse-width measurement is accurate even if the counter is armed while

a pulse train is in progress. If a counter is armed while the pulse is in the

active state, it waits for the next transition to the active state to begin the

measurement.

Single Pulse-Width Measurement

With single pulse-width measurement, the counter counts the number of

edges on the Source input while the Gate input remains active. When the

Gate input goes inactive, the counter stores the count in a hardware save

software then reads the stored count.