Siemens SIMATIC S7 System Manual page 106

Programming concepts
5.2 Elements of the user program
Note
Because it is not possible to predict when the CPU might generate an interrupt, it is
desirable to limit the number of variables that are used both by the interrupt routine and
elsewhere in the program.
Use the local variable table of the interrupt routine to ensure that your interrupt routine
uses only the temporary memory and does not overwrite data used somewhere else in
your program.
There are a number of programming techniques you can use to ensure that data is
correctly shared between your main program and the interrupt routines. Refer to the
descriptions of the Interrupt instructions (Page 322).
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART