Section 7. Installation
splicing, measurements in a slow sequence may span across multiple-scan
intervals in the main program. When no measurements need to be spliced, the
slow-sequence scan will run independent of the main scan, so slow sequences
with no measurements can run at intervals ≤ main-scan interval (still in 10 ms
increments) without skipping scans. When measurements are spliced, checking
for skipped slow scans is done after the first splice is complete rather than
immediately after the interval comes true.
In sequential mode, all instructions in slow sequences are executed as they occur
in the program according to task priority.
Auto self-calibration is an automatic, slow-sequence scan, as is the watchdog task.
Read More See Auto Self-Calibration — Overview
(p. 89).
7.6.3.13.3 SubScan() / NextSubScan
SubScan() / NextSubScan are used in the control of analog multiplexers (Analog
Multiplexers — List
(p. 562)) or to measure analog inputs at a faster rate than the
program scan. SubScan() / NextSubScan can be used in a SlowSequenc /
EndSequence with an interval of 0. SubScan cannot be nested. PulseCount or
SDM measurement cannot be used within a sub scan.
7.6.3.13.4 Scan Priorities in Sequential Mode
Note Measurement tasks have priority over other tasks such as
processing and communication to allow accurate timing needed within
most measurement instructions.
A priority scheme is used in sequential mode to avoid conflicting use of
measurement hardware. As illustrated in figure Sequential-Mode Scan Priority
Flow Diagrams
(p. 157), the main scan sequence has the highest priority. Other
sequences, such as slow sequences and auto self-calibration scans, must wait to
access measurement hardware until the main scan, including measurements and
processing, is complete.
Main Scans
Execution of the main scan usually occurs quickly, so the processor may be idle
much of the time. For example, a weather-measurement program may scan once
per second, but program execution may only occupy 250 ms, leaving 75% of
available scan time unused. The CR800 can make efficient use of this interstitial-
scan time to optimize program execution and communication control. Unless
disabled, or crowded out by a too demanding schedule, self-calibration (see Auto
Self-Calibration — Overview
(p. 89)) has priority and uses some interstitial scan
time. If self-calibration is crowded out, a warning message is issued by the
CRBasic pre-compiler. Remaining priorities include slow-sequence scans in the
order they are programmed and digital triggers. Following is a brief introduction
to the rules and priorities that govern use of interstitial-scan time in sequential
mode. Rules and priorities governing pipeline mode are somewhat more complex
and are not expanded upon.
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