Campbell Scientific SDM-INT8 8-Channel Interval Timer - Manual (Page 35)

Appendix B. Processing Time
Limitations
With the exception of the Capture All Events Option or when all channels are
programmed for low resolution frequency (Function 6), the INT8 is
performing two tasks simultaneously: (1) measuring and storing, and (2) doing
the processing necessary for the final calculations of averages. The time
required for processing varies depending on the total number of channels that
are used, the total number of edges that occur during the averaging interval,
and the function that each channel is programmed for.
With relatively high frequencies and particularly with the more involved
processing of the Counts on 2 Since 1 Function, the processing task can lag
behind the measuring and storing of the raw edge times. When averaging over
the execution interval or when doing continuous averaging (Output Options 0
and 0--) this can shorten the actual averaging window. When averaging over a
specified interval (Output Option XXXX) it can lengthen the execution time of
the INT8 instruction. The following formulas are used to calculate the Total
processing time (Tt).
Tt = To + ΣTi
To = Sampling Interval Overhead, ms
= 0.511*S +Et*(.034+.010*N)
Ti = processing Time for Function i, ms
T1 = 0.015*E1
T2 = 0.015*E2
T3 = 0.10*E3
T4 = 0.086*E4
T5 = 3.320*E5 +.116*Ec2
T6 = 0.015*E6
T7 = 0.015*E7
T8 = 0.1*E8 + .116*Ec2
where:
N = Number of channels measured.
Ei = Total number of edges occurring during the interval on those
channels programmed with Function i (i = 1...8).
Et = Total number of edges during the interval on all channels.
Ec2 = Edges occurring during the interval on channel 2.
S = The sampling interval in ms over which the INT8 samples the
channels.
If the Tt for Output Options 0 and 0-- is greater than the sampling interval, the
communication time with the INT8 will be extended from the minimum time
(2.3 ms +1.65 ms/value) only by the processing time needed to obtain an
answer for each channel, i.e., till at least one measurement is obtained. These
Options should have data ready immediately except in the case of a very slow
signal mixed in with relatively fast signals.
B-1