Chapter 9 Counters

 

Table 9-3summarizes some of the differences in methods of measuring

 

frequency.

 

 

 

 

Table 9-3.Frequency Measurement Method Comparison

 

 

 

 

 

 

 

 

 

Measures High

Measures Low

 

 

Number of

Frequency

Frequency

 

Number of

Measurements

Signals

Signals

Method

Counters Used

Returned

Accurately

Accurately

 

 

 

 

 

 

 

 

 

 

1

1

1

Poor

Good

 

 

 

 

 

1b

1

Many

Fair

Good

 

 

 

 

 

2

1 or 2

1

Good

Poor

 

 

 

 

 

3

2

1

Good

Good

 

 

 

 

 

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Position Measurement

You can use the counters to perform position measurements with quadrature encoders or two-pulse encoders. You can measure angular position with X1, X2, and X4 angular encoders. Linear position can be measured with two-pulse encoders. You can choose to do either a single point (on-demand) position measurement or a buffered (sample clock) position measurement. You must arm a counter to begin position measurements.

Measurements Using Quadrature Encoders

The counters can perform measurements of quadrature encoders that use X1, X2, or X4 encoding. A quadrature encoder can have up to three channels—channels A, B, and Z.

X1 Encoding

When channel A leads channel B in a quadrature cycle, the counter increments. When channel B leads channel A in a quadrature cycle, the counter decrements. The amount of increments and decrements per cycle depends on the type of encoding—X1, X2, or X4.

© National Instruments Corporation

9-15

NI USB-621x User Manual