Campbell Scientific SDM-CD8S 8 Channel Solid State DC Control Module - Manual (Page 25)

SDM-CD8S 8 Channel Solid State DC Control Module
EndIf
Site = SiteSequence(SeqIndex)
EndIf
SiteValveCD16
= SiteValve(Site)
EndOmit = Count > OmitCounts
AvgDisable = NOT (EndOmit AND SeqActiveFlag)
NextScan
EndProg
8.4 Control Gas Sampling with Timing in Measurement Task –
CR1000
8.4.1 The Purpose of this Example
This example illustrates an instruction that can be used if:
1) There is a requirement for precisely timed switching in a known timing
sequence.
2) The time required in the datalogger program for processing tasks is close to
the scan interval.
If the processing occasionally takes longer than the scan interval, there could
be skipped measurements (sequential mode) or the measurements could be
made but the switching would take place later than expected (pipeline mode).
This program has the same timing and valve control as the example in Section
8.2, however, instead of the logic in the program determining when to switch
the valves, it makes use of the TimedControl instruction to control when valves
switch as part of the measurement task (pipeline mode only).
8.4.2 Pipeline vs Sequential
The Pipeline mode is more efficient than the Sequential mode. In the
Sequential mode, all processing is completed before advancing to the next
instruction. The Pipeline mode separates the measurement and processing
tasks. Not only does this separation of tasks allow the datalogger to complete
both tasks in less time than the sequential mode, it allows the processing task
to briefly fall behind the measurement task without skipping any
measurements.
In the pipeline mode the results of the measurement task are stored in a buffer
for the processing task. The processing task starts after the first measurement
task is complete and can take place at the same time as the next measurement
task. If the processing task has something that temporarily prevents it from
finishing before the next measurement scan is complete, it can fall behind the
measurement scan. Later when the processing is less intensive, the processing
can run faster than the measurements and process the buffered measurement
data and catch up to the measurement scan. Operations that may temporarily
increase the processing are intensive calculations for final output processing,
extra communications overhead, or initializing a new compact flash card.
19