Memory Backups

Cannot believe some DCS/PLC still use batteries. Yes I know all about dual power supplies and UPS, but to use this primitive method! We usually specify EEPROM but the ABB800 got passed us!. The Bailey N90, still the greatest system out was using EEPROM in 1980.

When battery backup memory were common the advantage they had over non-volatile memory was the very large number and speed of the write cycles they can perform. Most processor machines (including a PLC) properly infrequently write to non-volatile memory locations by better OS control. The problem of any battery supported memory is that everyone forgets the battery until it fails.

Backup in nVSRAM is fastest way to save changes. EEPROMs are slow in saving data and also have life for the number of times they can save data. However, nVSRAM require small Lithoum cell to protect data for about 5 to 10 years. Some EEPROM may permit million write operations so they are not all that bad but do have slow byte or page writing methods. One can use Flash if entire program is changed. EPROM or PROM or OTP if you do not want anyone to change program.

Recently FRAM have come to the market are like SRAM in speed and also save data automatically on power down. These may enter into PLCs very soon.