Cpu Memory - Siemens SIMATIC S7 System Manual

PLC concepts
3.1 Execution of the user program
Configuring the cycle time and communication load
You use the CPU properties in the Device configuration to configure the following
parameters:
● Cycle time: You can enter a maximum scan cycle time. You can also enter a fixed
● Communications load: You can configure a percentage of the time to be dedicated for
For more information about the scan cycle, see "Monitoring the cycle time". (Page 43)
3.1.3

CPU memory

Memory management
The CPU provides the following memory areas to store the user program, data, and
configuration:
● Load memory is non-volatile storage for the user program, data and configuration. When
● Work memory is volatile storage for some elements of the user project while executing
● Retentive memory is non-volatile storage for a limited quantity of work memory values.
To display the memory usage for the current project, right-click the CPU (or one of its blocks)
and select "Resources" from the context. To display the memory usage for the current CPU,
double-click "Online and diagnostics", expand "Diagnostics", and select "Memory".
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minimum scan cycle time.
communication tasks.
a project is downloaded to the CPU, it is first stored in the Load memory area. This area
is located either in a memory card (if present) or in the CPU. This non-volatile memory
area is maintained through a power loss. The memory card supports a larger storage
space than that built-in to the CPU.
the user program. The CPU copies some elements of the project from load memory into
work memory. This volatile area is lost when power is removed, and is restored by the
CPU when power is restored.
The retentive memory area is used to store the values of selected user memory locations
during power loss. When a power down occurs, the CPU has enough hold-up time to
retain the values of a limited number of specified locations. These retentive values are
then restored upon power up.
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02