Vernier LabPro - Manual (Page 33)

Revision Date: 08/02/02
LabPro Technical Manual 33
Syntax: {1, channel, operation, list of values}
Channel Operation List of Values
31 = Digital Output 1
32 = Digital Output 2 0 = Clears the channel
1-32 = Count (number of data elements in list) Values may be from 0 to 15.
The list of values must have one value for each count.
Example 5: Count down from 5. Note we set Channel 1 active. We don't have to retrieve the data.
Computer
Calculator
s{102,-1} :Send({102,-1})
s{1,31,0} :Send({1,31,0})
s{1,31,6,5,4,3,2,1,0} :Send({1,31,6,5,4,3,2,1,0})
Set the period
Start counter
Measuring Period and Frequency
Period and frequency apply only to CH1 and only CH1 can be active if the operation is set to 5 (Period) or
6 (Frequency). Period and frequency are measured on Vin pin (pin 1) of CH1. Period and frequency
measurements always use the hardware threshold.
LabPro measures period and frequency by counting edges for 0.25 seconds, or by measuring the time
between the selected edges for one period—whichever is larger (see figure below). If a significant number
of edges are counted during the 0.25-second period, the count is used to compute both period and
frequency; otherwise, the period and frequency are computed from the time interval for one period.
0 1 2 3 T=
+
-
+
-
Figure 2. Period and Frequency Measurement
Trigger Type Measuring Points
2
3
4
5
+ + (T=0 to 2)
- - (T=1 to 3)
+ +- (T=0 to 1)
+ -+ (T=1 to 2)
The crossover point between the two computations is about 600 Hz. Because there can be a one-count
uncertainty during the 0.25-second period, the accuracy around 600 Hz is approximately ±4 Hz (about
0.7%). The resolution of the timer measuring the time between edges is 6.4 microseconds; therefore, the
percentage accuracy improves for frequencies above and below 600 Hz.
If LabPro is set up using Command 3 to make multiple measurements at a particular sample time, LabPro
waits for the sample time that you specified after it completes the current measurement. It then initiates the
next cycle of period/frequency measurement. The minimum sampling time for period and frequency is
0.25 seconds.
Note: Period and frequency measurements using Trigger Type 4 or 5 are only possible on non-repetitive
signals or on repetitive signals that are less then 600 Hz. This is because at 600 Hz, the edge counts will
prevail.