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What is the cause of the error message "error writing to cache file: AlarmGroup1" when booting a Magelis HMI?

Published date: 05 November 2018

The "cache file" that the error is referring to is created on the secondary or optional drive under the following conditions:
1. "Backup Alarm Group" is enabled.
2. "Dynamic Message" is enabled.
3. There is at least one alarm that has a dynamic message configured.

If there is no secondary or optional drive connected to the target, then the behaviour is normal.

The purpose of the alarm cache file is to store dynamic alarms so that they are not lost when the target is rebooted. When an alarm occurs, it is stored in SRAM. Eventually, the SRAM buffer becomes full as more alarms occur during runtime. When SRAM becomes full, the older dynamic alarms are written to the alarm cache file on the secondary or optional drive to make room in SRAM for new alarms.

When you reboot the target, Runtime loads the first x alarm records from SRAM, and the rest from the alarm cache file, where x is the value of "Num of History Alarms" or "Num of Log Alarms" in the project.

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