What types of memory are used in PLCs?

PLC memory types include: ROM (Read Only Memory) - stores operating system and firmware. RAM (Random Access Memory) - stores program and data during operation, volatile. Flash/EEPROM - non-volatile program storage, retains program on power loss. Battery-backed RAM - retains runtime data (timers, counters) during power loss. Memory organization: system memory (OS), program memory (user logic), data memory (I/O images, variables), and communication buffers. Modern PLCs have expandable memory via SD cards.