Memory Concept - Siemens SIMATIC S7-400 Configuration And Use Manual

2.3

Memory Concept

Introduction
The CPUs of the S7-400 have the following types of memory:
• Loading memory for project data, for example, blocks, configuration and
parameter assignment data including symbols and comments.
• User memory for the runtime-relevant blocks (code blocks and data blocks) and
the system memory.
Loading Memory
The loading memory consists of a plug-in memory card (RAM or FLASH EPROM).
An integrated RAM is provided for test purposes or for reloading/correcting blocks
when a FLASH EPROM memory card is used.
Additional information is stored in the loading memory along with the project data.
This might include format identifications, symbols and comments, to enable full
decompiling of the user program with any programming device. The loading
memory also stores the parameter blocks for other modules used in the
automation system.
User Memory
The user memory (integrated RAM) is used to hold the parts of the user program
that are relevant for executing the program. The program is executed exclusively in
the area of the user and system memory.
System Memory
The system memory contains the memory elements that each CPU makes
available to the user program, such as the process image of the inputs and outputs
and local data.
Buffering
The CPUs of the SIMATIC S7-400 are buffered by the power supply module,
enabling them to retain their entire memory contents if a power failure occurs. The
power supply modules feature either one or two batteries. They can also be
operated with a back-up battery, however.
The CPUs can also be backed up by an external power source.
Retention of Timers, Counters, Memory Bits and Data Blocks
Timers, counters, memory bits and data blocks can retain their old values even
after reboot. All you have to do for this is to define these operands as non-volatile
using STEP 7.
Automation System S7-400 Configuration and Use
A5E00442711-02
CPUs of the S7-400
2-9