Memory Requirements - Siemens SIMATIC S7-400 Reference Manual

Structure of a CPU 41x
RAM Card
you use a RAM card, you must plug this into the CPU to load the user program.
The user program is loaded with the help of the programming device (PG).
You can load the entire user program or the individual parts such as FBs, FCs,
OBs, DBs, or SDBs into the load memory in STOP mode or in RUN-P mode.
If you remove the RAM card from the CPU, the information stored on it is lost. The
RAM card does not have a built-in backup battery.
If the power supply has a functioning backup battery or if an external backup
voltage is supplied to the CPU via the "EXT. BATT." socket, the contents of the
RAM card are retained after switching off the power supply provided the RAM card
remains plugged into the CPU and the CPU remains in the rack.
Flash Card
If you use a Flash card, there are two ways of loading the user program:
• Set the CPU to STOP with the mode selector, plug the Flash card into the CPU,
and load the user program with
STEP 7 "PLC –> Load User Program to Memory Card".
• Load the user program into the Flash card in offline mode at the programming
device or adapter and then insert the Flash card into the CPU.
You can only load your complete user program with the Flash card. You can load
smaller program sections into the integrated load memory on the CPU using the
programming device. In the case of larger program changes, you must always
reload the Flash card with the complete user program.
The Flash card does not require voltage to store its contents, that is, the
information stored on it is retained even when you remove the Flash card from the
CPU or if you operate your S7-400 system without backup (without backup battery
in the power supply module or "EXT. BATT." socket of the CPU).
Which Memory Card Capacity to Use
The capacity of the memory card you use depends on the size of the user program
and the additional memory requirement resulting from the use of function modules
or communications modules. See the manuals of these modules for details of their

memory requirements.

To optimally use the working memory (code and data) your CPU, you should
expand the load memory of the CPU with a memory card with at least the same
capacity as the working memory.
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Automation System S7-400 CPU Specifications
A5E00165965-01