The Siemens 6ES7951-1KK00-0AA0 is a memory card for the S7-300 PLC family, providing 1 Mbyte of Flash-EPROM storage. Flash-EPROM means the program and data are retained without battery backup and can be rewritten in the field — no UV eraser needed. The 1 Mbyte capacity is sized for medium-complexity S7-300 programs; a larger CPU or external memory card would be needed for extensive function blocks or data logging.
This card plugs directly into the memory-card slot on any S7-300 CPU (e.g., CPU 315-2 DP, CPU 317-2 PN/DP). No tools, no wiring. The short-design form factor keeps it flush with the CPU front panel, so it does not obstruct adjacent modules in a densely populated rack. The 5V Flash-EPROM interface is standard across the S7-300 line — no adapter needed.
What the 1 Mbyte rating means for your program
The 1 Mbyte capacity is the total usable memory for user program blocks (OB, FB, FC, DB) and system data. In practice, a 1 Mbyte card can hold roughly 10,000 to 15,000 instructions of ladder or STL, depending on data block size. If your S7-300 project already compiles to under 900 Kbytes, this card fits without margin concerns. If you are near the limit, step up to the 2 Mbyte or 4 Mbyte variant in the same short-design family.