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    Effect of Memory Protect switch position on Controller behavior.

    Goals and Symptoms


    The purpose of this resolution is to describe the effect of the Memory Protect switch position on CPU behavior.

    Facts and Changes

    The following table describes the behavior of the CPU with various Mem Protect switch and logic conditions.

     
    CPU State
    Mem Protect
    Switch Position
    Behavior
    Dim Awareness
    OFF
    Program will load from flash and go into RUN
    Dim Awareness
    ON
    CPU will not go into run as logic in RAM does not exist.
    Program in Flash
    Modified Program in RAM
    OFF
    Flashed program will load overwritng RAM program
    Program in Flash
    Modified Program in RAM
    ON
    Modified RAM program will load and program in flash is ignored.
     
    Causes and Fixes

    Here are a few scenarios:


    Download a project to the CPU and follow up by saving it to flash.
    Flash and RAM are now identical.

    With the Memory Protect switch in the OFF position, you login and make changes to the program online.
    These changes are only saved in RAM and not in the flash area of memory.
    If you want to save these changes, the CPU must be stopped and the program is then saved to flash.

    You have a power failure and you want the latest version of the program to load.
    The memory protect switch is ON and the program in RAM will load and start.
    Flash RAM contents are ignored.

    You have a power outage and the battery in the PLC has failed, effectively clearing out all contents of RAM.
    Recovery is done by placing the memory protect switch to the OFF position and cycling power.
    The program will now load from Flash to RAM.

    Recommendations:

    For day in/out operation, leave the memory protect switch OFF, log in and make changes to the program.
    When you are done, simply turn the memory protect switch ON.
    If you get a power event, the system will restart with RAM data rather the Flash contents.

    Summary:

    Memory Protect is protecting the contents of RAM.
    If Memory Protect is ON during a power cycle, flash will not overwrite RAM.
    If Memory Protect is OFF, flash will overwrite RAM on a power cycle.

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