Technical data

Appendix
27.2 Memory Areas of S7 CPUs
Programming with STEP 7
Manual, 05/2010, A5E02789666-01 577
in it. Writing the process image output table to the outputs for the modules and reading in the
process image input table is all independently controlled by the operating system.
Advantages of the Process Image
Compared with direct access to the input/output modules, the main advantage of accessing the
process image is that the CPU has a consistent image of the process signals for the duration of
one program cycle. If a signal state on an input module changes while the program is being
executed, the signal state in the process image is retained until the process image is updated again
in the next cycle. The process of repeatedly scanning an input signal within a user program
ensures that consistent input information is always available.
Access to the process image also requires far less time than direct access to the signal modules
since the process image is located in the internal memory of the CPU.
Part Process Images (Process-Image Partitions)
In addition to having the process image (process-image input table, PII, and process-image output
table, PIQ) automatically updated by the operating system, you can assign parameters to a
maximum of 15 partial process images for an S7-400 CPU (CPU-specific, no. 1 to no. 15, see the
S7-400, M7-400 Programmable Controllers Module Specifications Reference Manual). This means
that you can update sections of the process-image table, when necessary, independently of the
cyclic updating of the process image table.
Each input/output address that you assign with STEP 7 to a process-image partition no longer
belongs to the OB1 process-image input/output tables. Input and output address can only be
assigned once through the OB 1 process image and all process-image partitions.
You define process-image partition with STEP 7 when you assign addresses (which input/output
addresses of the modules are listed in which process-image partition). The process-image partition
is updated either by the user with SFCs or automatically by the system by connecting to an OB.
Exception: Process image partitions for synchronous cycle interrupt OBs are not updated on the
system side, even though they are linked to an OB (OB 61 to OB 64).
Note
For S7-300 CPUs, unassigned process-image inputs and outputs can be used as additional bit
memory areas. Programs that use this capability can run on older (that is, before 4/99) S7-400 CPUs
only under one of the following conditions:
For these S7-400 CPUs
• The process image areas used as bit memory must be located outside of the parameter assignment for "Size
of the Process Image" or.
• must be located in a process-image partition that is updated neither by the system nor by SFC26/SFC27.