Introduction to memory classification of PLC system
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How is PLC memory classified?
Three primary types: RAM (volatile, holds the actively changing data such as sensor inputs, timer values and counter states; contents lost on power failure unless battery-backed), ROM (non-volatile, holds the firmware and operating system permanently programmed at the factory), and EEPROM (non-volatile but user-writable, holding programs, recipes and setpoints that must survive power cycles; has a limited write-cycle lifespan). Current controllers are replacing battery-backed RAM with battery-free retention technologies such as MRAM and FRAM.
The three memory types
| Type | Volatility | Primary function | Characteristics |
|---|---|---|---|
| RAM | Volatile (requires power or battery) | Data memory and active program execution | Stores constantly changing values; contents lost on power failure unless backed |
| ROM | Non-volatile | Firmware and system OS storage | Programmed at the factory; retains boot instructions without power |
| EEPROM | Non-volatile | User programs, recipes, setpoints | User-writable and erasable; limited write-cycle lifespan |
Scroll the table horizontally to view all columns.
Source: general PLC memory architecture per manufacturer documentation. Current families may substitute FRAM or MRAM for EEPROM; confirm in the CPU model's selection data.
Retention and the battery question
The RAM type is the one that drives the battery question: if the PLC's data memory is volatile, the installation needs a battery or capacitor to preserve retentive values during power-off. Current families are replacing battery-backed SRAM with battery-free alternatives (MRAM, FRAM, supercap-backed SRAM) that survive power cycles without a maintenance item. Check which scheme your CPU generation uses, because it changes what belongs on the spares shelf: batteries for legacy, nothing for the newer families.
Sourcing help
KOEED supplies PLC CPUs, memory modules and retention batteries across the major brands, in active and EOL stock. Send the CPU model for a quote.
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