Chapter 8
Counters
©
National Instruments Corporation
8-15
•
Method 2 is acc
u
rate for high freq
u
ency signals. However, the
acc
u
racy decreases as the freq
u
ency of the signal to meas
u
re
decreases. At very low freq
u
encies, Method 2 may be too inacc
u
rate
for yo
u
r application. Another disadvantage of Method 2 is that it
req
u
ires two co
u
nters (if yo
u
cannot provide an external signal of
known width). An advantage of Method 2 is that the meas
u
rement
completes in a known amo
u
nt of time.
•
Method 3 meas
u
res high and low freq
u
ency signals acc
u
rately.
However, it req
u
ires two co
u
nters.
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
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.
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
