3. S7 PLC DATA & MEMORY

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3. PLC DATA & MEMORY
Here we will illustrate the type of data
which PLC deals with its range, symbols,
addresses…….etc.

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Memory Areas of S7 CPUs
Distribution of the Memory Areas
The memory of an S7 CPU can be divided into three areas (see figure below):
1. The load memory is used for user programs without symbolic address assignments or
comments (these remain in the memory of the programming device) The load memory
can be either RAM or EPROM
• Blocks that are not marked as required for startup will be stored only in the load memory.
2. The work memory (integrated RAM) contains the parts of the S7 program relevant for
running your program, the program is executed only in the work memory and system
memory areas.
3. The system memory (RAM) contains the memory elements provided by every CPU for
the user program, such as the process-image input and output tables, bit memory, timers,
and counters. The system memory also contains the block stack and interrupt stack.
• In addition to the areas above, the system memory of the CPU also provides temporary
memory (local data stack) that contains temporary data for a block when it is called, This
data only remains valid as long as the block is active.
Load Memory and Work Memory
When you download the user program from the programming device to the CPU, only the logic
and data blocks are loaded in the load and work memory of the CPU.

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The symbolic address assignment (symbol table) and the block comments remain on the
programming device.
Dividing Up the User Program
To ensure fast execution of the user program and to avoid unnecessary load on the work memory
that cannot be expanded, only the parts of the blocks relevant for program execution are loaded
in the work memory.
Parts of blocks that are not required for executing the program (for example, block headers)
remain in the load memory.
The following figure shows a program being loaded in the CPU memory.
Note
Data blocks that are created in the user program with the help of system functions (for example,
SFC22 CREAT_DB) are saved entirely in the work memory by the CPU.
Some CPUs have separately managed areas for code and data in the work memory. The size and
assignment of these areas is shown in the "Memory" tab of the Module Information for these
CPUs.
Load Memory Structure
The load memory can be expanded using memory cards. Refer to your "S7-300 Programmable
Controller, Hardware and Installation Manual" and your "S7-400, M7-400 Programmable

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Controllers Module Specifications Reference Manual" for the maximum size of the load
memory.
The load memory can also have an integrated EPROM part as well as an integrated RAM part in
S7-300 CPUs. Areas in data blocks can be declared as retentive by assigning parameters in STEP
7 (see Retentive Memory Areas on S7-300 CPUs).
In S7-400 CPUs, it is imperative that you use a memory card (RAM or EPROM) to expand the
load memory. The integrated load memory is a RAM memory and is mainly used to reload and
correct blocks. With the new S7-400 CPUs, additional work memory can also be plugged in.
• Programs stored in RAM are lost when you reset the CPU memory (MRES) or if you
remove the CPU or RAM memory card.
• Programs saved on EPROM memory cards are not erased by a CPU memory reset and
are retained even without battery backup (transport, backup copies).
System Memory
The system memory of the S7 CPUs is divided into address areas (see table below). Using
instructions in your program, you address the data directly in the corresponding address area.

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Email: [e-mail] ٢٠ عر ٢٩٥ - ىدا ةا - ى ر١١٤٣٥ - ت : ٢٥١٨١١٣٨/٠٢ ٠٠٢ ف :٢٥١٨١١٣٧/٠٢ ٠٠٢ 27