User data memory, user program memory and system configuration memory on a Programmable Logic Controller (PLC) can be backed up in any of four different ways:
* battery capacitor
* lithium battery
* flash memory
* memory cards
If a battery capacitor assembly is used, it automatically backs up the current memory values in the event of a power loss. When power is restored, the PLC comes back up operational with the configuration and program values that were present at the time power was lost. The battery capacitor can back up to 72 hours typically.
If a lithium battery assembly is used, memory data will safely be backed up to at least 5 years. However, consider installing a fresh battery if the battery has not been changed recently and the PLC has been powered down for a period of more than ten days.
A portion of the flash memory in all PLCs is reserved for storing the system configuration, user logic and user data memory. This feature backs up the configuration and user logic even if you do not use a battery capacitor or lithium battery. If memory is restored to the PLC from flash memory after a power loss, the values that were current at the time of your last save operation will be restored.
The memory card is a recent method for memory backup. Some new PLCs provide a slot for a memory card. The PLC ladder application program can read and write to the memory card. The memory data is saved as a comma separated value file (.csv), which is compatible with several PC applications such as Excel. The PLC memory cards follow the DOS/Windows standard FAT-16 file system. All names must be limited to the â€˜eight dot three’ format where the filename contains eight characters, a period then a three-character extension.
Power-up procedures for PLCs follow a series of logical steps. When the PLC receives power, it first checks system configuration memory to see if a valid configuration exists. If a valid configuration has been saved via the optional battery backup, these values will be present in user data memory. The PLC will configure itself with these values and be ready to operate.
If the PLC does not detect a valid configuration in user data memory, it will check the flash memory backup. If a valid configuration has been saved in flash memory, the PLC will configure itself with these values and be ready to operate. If the PLC cannot find a valid configuration in data memory or flash memory, it will power up in an unconfigured condition. You need to insert a memory card with an application program or connect a programming panel to the PLC and configure it before it can be programmed or before it can solve logic.