76 Rockwell Automation Publication 2080-UM005E-EN-E - March 2018
Chapter 6 Program Execution in Micro800
Power Up and First Scan
In Program mode, all analog and digital input variables hold their last state, and
the LEDs are always updated. Also all analog and digital output variables hold
their last state, but only the analog outputs hold their last state while the digital
outputs are off.
When transitioning from Program mode to Run mode, all analog output
variables hold their last state but all digital output variables are cleared.
Two system variables are available.
Variable Retention
Micro830 and Micro850 controllers retain all user-created variables after a power
cycle, but the variables inside instances of instructions are cleared. For example: A
user created variable called My_Timer of Time data type will be retained after a
power cycle but the elapsed time (ET) within a user created timer TON
instruction will be cleared.
Unlike Micro830/Micro850 controllers, Micro810® and Micro820 controllers
can only retain a maximum of 400 bytes of user-created variable values. This
means that after a power cycle, global variables are cleared or set to initial value,
and only 400 bytes of user-created variable values are retained. Retained variables
can be checked at the global variable page.
System Variables for Scan and Powerup
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.
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