M90/91 and Jazz controllers have an internal battery back-up for certain values during a power failure.
M90 models
The battery backs up values from:
MI 0 - MI 15
MB 0 - MB 15
RTC value
This means that only these values are saved if the controller is powered off. All other data types, including database registers, are initialized.
Therefore, any Operand that must retain its value during a power failure must be written into one of the above Operands.
M91/Jazz models
All system data, including Database and the RTC value are backed up.
You can control the back-up feature via Download options:
The Retain Values Battery Backup option. This is checked by default. This option saves all values, including Database register values, in the event that the controller is powered off.
Initialize Data Types Options excluding MI & MB 0-15 (M90 style). If you select this option, and the controller is powered off, only the values in MI & MB 0-15 and the RTC value are retained. All other data types, including database registers, are initialized. This makes the backup function like the M90. This may be useful in projects written initially for the M90.