Campbell CR800 Specifications download pdf (Page 257)

Section 9. Program Control Instructions

Slow Sequence

The SlowSequence instruction is used to mark the beginning of a section of

code that will run concurrently with the main program.

Syntax

SlowSequence

Remarks

The SlowSequence statement marks the end of the main program and begins a

separate sequence of instructions. The instructions for the slow sequence

program are executed when the main program is not running as time allows. It

is possible to have up to four slow sequences executing at a rate different than

that of the primary scan interval. Slow sequences can be declared with a

Scan/NextScan structure, or they can be placed within a Do/Loop to execute

whenever the datalogger is not busy with other tasks.

Because measurements in the main scan have priority over all other tasks, the

measurement instructions in a slow sequence are performed during the times

when the datalogger is not running the main scan. This can result in the

measurements of a slow sequence being performed over a longer period of

time than if they were placed in the main scan.

Priority of a slow sequence in the datalogger will vary, depending upon

whether the datalogger is executing its program in pipeline mode or sequential

mode. With the important exception of measurements, when running in

pipeline mode all sequences in the program have the same priority. When

running in sequential mode, the main scan has the highest priority for

measurements, followed by background calibration (which is automatically run

in a slow sequence), then the first slow sequence, the second slow sequence,

and so on. Refer to section OV 2.3 for additional information on the priority of

sequences in the datalogger.

Slow sequences are typically run at a slower rate than the main program. They

can be run at a faster rate if there are no measurement instructions in the slow

sequence. There is, however, a risk of skipping scans in a slow sequence if the

main scan interval is set too fast. A rule of thumb is that the main scan should

be no faster than N + 1 * the fastest slow sequence, where N is the number of

slow sequences in the program and the “1” is to account for background

calibration. For example, if there are three slow sequences in the program, the

main scan interval should be four times faster than the fastest slow sequence.

Subroutines and data tables called by a slow sequence should be declared after

the SlowSequence instruction. Data written to data tables within a slow

sequence will be time stamped with the start time of the slow sequence scan.

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Campbell CR800 Specifications Page 257