Rockwell Automation 2080-LC50 Micro830 and Micro850 Programmable Controllers User Manual - Manual (Page 71)

Rockwell Automation Publication 2080-UM002F-EN-E - December 2013 57
Program Execution in Micro800 Chapter 6
marginally. The Watchdog setting defaults to 2 s and generally never needs to be
changed.
Periodic Execution of Programs
For applications where periodic execution of programs with precise timing is
required, such as for PID, it is recommended that STI (Selectable Timed
Interrupt) be used to execute the program. STI provides precise time intervals.
It is not recommended that the system variable __SYSVA_TCYCYCTIME be
used to periodically execute all programs as this also causes all communication to
execute at this rate.
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 from revision 2 and later.
WARNING: Communication timeouts may occur if programmed cycle
time is set too slow (for example, 200 ms) to maintain communications.
System Variable for Programmed Cycle Time
Variable Type Description
__SYSVA_TCYCYCTIME TIME Programmed cycle time.
Note: Programmed cycle time only accepts values in
multiples of 10 ms. If the entered value is not a
multiple of 10, it will be rounded up to the next
multiple of 10.
System Variables for Scan and Powerup on Firmware Release 2 and later
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, it is false.
_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, microSD card).
Note:True only on the first scan after a powerup, or
running a new ladder for the first time.