Rockwell Automation 2080-LC10-12DWD Micro810 Programmable Controllers User Manual - Manual (Page 43)

Rockwell Automation Publication 2080-UM001D-EN-E - September 2012 35
Program Execution in Micro800 Chapter 6
In a case where bindings are defined, variables consumed by a resource are
updated after the inputs are scanned and the variables produced for other
resources are sent before updating inputs.
When a cycle time is specified, a resource waits until this time has elapsed before
starting the execution of a new cycle. The POUs execution time varies depending
on the number of active steps in SFC programs and instructions such as jumps,
IFs, and returns. When a cycle exceeds the specified time, the loop continues to
execute the cycle but sets an overrun flag. In such a case, the application no longer
runs in real time.
When a cycle time is not specified, a resource performs all steps in the loop then
restarts a new cycle without waiting.
Power Up and First Scan
On firmware revision 2 and later, all digital output variables driven by the I/O
scan gets cleared on powerup and during transition to RUN mode.
Two system variables are also available on revision 2.x:
Periodic Execution of Programs
It is not recommended that the system variable __SYSVA_TCYCYCTIME be
used to periodically execute all programs as this also causes the communication to
execute at this rate.
System Variables for Scan and Powerup on Release 2.x
Variable Type Description
_SYSVA_FIRST_SCAN BOOL First scan bit.
Can be used to initialize or reset variables immediately
after every transition from Program to Run mode.
Note: True only on first scan. After that, go to run.
_SYSVA_POWER_UP_BIT BOOL Powerup bit.
Can be used to initialize or reset variables immediately
after download from Connected Components
Workbench or immediately after being loaded from
memory backup module (for example, 2080-MEMBAK-
RTC, 2080-LCD).
Note: True only on the first scan after a powerup, or
running a new ladder for the first time.
WARNING: Communication timeouts may occur if programmed cycle
time is set too slow (for example, 200 ms) to maintain communications.