Campbell Scientific SDM-INT8 8-Channel Interval Timer User Manual
Page 20
SDM-INT8 8 Channel Interval Timer
TABLE 3. Input Frequency (kHz) at Which Processing
Time Equals Measuring/Storing Time
N
1,2,6,7
3
4
5
8
1
*
3.4
3.7
0.14 - 0.05F2 3.4 - 0.81F2
2
3.5
1.6
1.7
0.07 - 0.02F2
1.6 - 0.38F2
3
2.1
0.99
1.1
0.05 - 0.02F2
0.99 - 0.24F2
4
1.4
0.70
0.76 0.04 - 0.01F2
0.70 - 0.17F2
5
.99
0.53
0.57 0.03 - 0.01F2
0.53 - 0.13F2
6
.75
0.42
0.45 0.02 - 0.01F2
0.42 - 0.10F2
7 .59
0.34
0.37
8 .47
0.28
0.30
N = Number of channels measuring given Function
* = Greater than the maximum input frequency of 5.1 kHz
F2 = Average input frequency on channel 2
See Appendix B to formulate the equations used to generate
Table 3. Frequencies show in Table 3 are for "worst case"
conditions. Faster input frequencies are possible depending on
the phase relationship of the channel to channel signal.
With Options 0, 32768, and 0--, the average returned to the datalogger is the
most recently processed average when the INT8 is addressed. If processing
lags measuring/storing, the number of samples used in the average is reduced
as is the effective averaging interval. For functions that average, this is not a
problem, assuming the input frequency does not change significantly over the
sampling interval. It is a problem if counts are being totalized (Function 7,
Output Option 32768 or 0--). In this case the count will intermittently be low
(Section 6.4.3).
The Specified Averaging Interval Option (nnnn or XXXX) uses all events
captured over the specified interval to calculate an average. If the processing
tasks gets behind the measuring/storing task, the additional time required to
process all the edges is taken at the expense of the execution time (refer to
Table 3.)
Due to finite memory in the INT8, when processing lags behind by 800 edges,
the measuring/storing task is suspended for that interval. For Option nnnn or
XXXX to average over the entire specified interval, the interval must be short
enough to prevent the processing tasks from getting behind by more than 8000
edges. Table 4 gives the sampling interval at which 8000 unprocessed events
will accumulate for a given input frequency.
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