Sizing the program store for an S7-300 CPU
The Siemens 6ES7951-0KG00-0AA0 is a Flash-EPROM memory card for the S7-300 family, providing 128 kbyte of non-volatile program and data storage. It plugs directly into the CPU slot and holds the user program, data blocks, and configuration — the CPU loads from it at power-up, so the program survives a battery failure or a card swap. For a typical S7-300 application, 128 KB covers medium-sized logic with a few dozen function blocks and a modest data block set; if your project exceeds that, you step up to the 256 KB or 512 KB cards in the same form factor.
Flash-EPROM vs RAM — why it matters for uptime
This is Flash-EPROM, not battery-backed RAM. That means the program stays resident without a backup battery on the CPU — useful in a panel where battery changes are infrequent or the CPU sits in a remote location. The trade-off: Flash-EPROM has a finite write-cycle endurance (typically 100,000 cycles), so it is not suited for frequent online rewrites during commissioning. For production machines where the program is stable after initial download, it is the right choice. The mature lifecycle stage means Siemens no longer actively develops this card, but it remains available through distribution channels for installed-base support.