Siemens SIMATIC ET 200S Manual page 134

Functions of the IM 151-7 CPU
Local data
The following are stored as local data:
S The temporary variables of code blocks
S The start information of the organization blocks
S Transfer parameters
S Intermediate results
Temporary variables
When creating blocks, you can declare temporary variables (TEMP) which are only
available while the block is being processed and are then overwritten again. This
local data has a fixed length for each OB. The local data must be initialized before
the first read access operation. Furthermore, each organization block requires 20
bytes for its start information. Local data is accessed faster than data in the DBs.
The IM 151-7 CPU has memory for the temporary variables (local data) of blocks
that have just been processed. This memory is divided equally between the priority
classes. Each priority class has its own local data area.
Caution
!
All temporary variables (TEMP) of an OB and its subordinate blocks are stored in
the local data. The local data area could overflow if you use a large number of
nesting levels for block processing.
The IM 151-7 CPU changes to the STOP mode if you exceed the permitted
quantity of local data in a priority class.
Take into consideration the amount of local data required by the synchronous fault
OBs. In each case, the local data requirement is assigned to the responsible
priority class.
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ET 200S IM 151-7 CPU Interface Module
A5E00058783-04