National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 8 Counters
© National Instruments Corporation 8-15 NI USB-621x User Manual
Table 8-2 s u mmarizes some of the differences in methods of meas u ring
freq u ency.
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Position Measurement
Yo u can u se the co u nters to perform position meas u rements with
q u adrat u re encoders or two-p u lse encoders. Yo u can meas u re ang u lar
position with X1, X2, and X4 ang u lar encoders. Linear position can be
meas u red with two-p u lse encoders. Yo u can choose either a single point
(on-demand) position meas u rement or a b u ffered (sample clock) position
meas u rement. Yo u m u st arm a co u nter to begin position meas u rements.
Measurements Using Quadrature Encoders
The co u nters can perform meas u rements of q u adrat u re encoders
that u se X1, X2, or X4 encoding. A q u adrat u re encoder can have u p
to three channels—channels A, B, and Z.
• X1 Encoding —When channel A leads channel B in a q u adrat u re
cycle, the co u nter increments. When channel B leads channel A in a
q u adrat u re cycle, the co u nter decrements. The amo u nt of increments
and decrements per cycle depends on the type of encoding—X1, X2,
or X4.
Fig u re 8-14 shows a q u adrat u re cycle and the res u lting increments and
decrements for X1 encoding. When channel A leads channel B, the
increment occ
u rs on the rising edge of channel A. When channel B
leads channel A, the decrement occ u rs on the falling edge of
channel A.
Table 8-2. Frequency Measurement Method Comparison
Method
Number of
Counters
Used
Number of
Measurements
Returned
Measures High
Frequency
Signals
Accurately
Measures Low
Frequency
Signals
Accurately
1 1 1 Poor Good
1b 1 Many Fair Good
2 1 or 2 1 Good Poor
3 2 1 Good Good