Siemens Simatic S7-200 System Manual page 303

Sharing Data Between the Main Program and Interrupt Routines
You can share data between the main program and one or more interrupt routines. For
example, a part of your main program may provide data to be used by an interrupt routine, or
vice versa. If your program is sharing data, you must also consider the effect of the
asynchronous nature of interrupt events, which can occur at any point during the execution
of your main program. Problems with the consistency of shared data can result due to the
actions of interrupt routines when the execution of instructions in your main program is
interrupted by interrupt events.
There are a number of programming techniques you can use to ensure that data is correctly
shared between your main program and interrupt routines. These techniques either restrict
the way access is made to shared memory locations, or prevent interruption of instruction
sequences using shared memory locations.
For an STL program that is sharing a single variable: If the shared data is a single byte,
word, or double-word variable and your program is written in STL, then correct shared
access can be ensured by storing the intermediate values from operations on shared
data only in non-shared memory locations or accumulators.
For a LAD program that is sharing a single variable: If the shared data is a single byte,
word, or double-word variable and your program is written in ladder logic, then correct
shared access can be ensured by establishing the convention that access to shared
memory locations be made using only Move instructions (MOV_B, MOV_W, MOV_DW,
MOV_R). While many LAD instructions are composed of interruptible sequences of STL
instructions, these Move instructions are composed of a single STL instruction whose
execution cannot be affected by interrupt events.
For an STL or LAD program that is sharing multiple variables: If the shared data is
composed of a number of related bytes, words, or double-words, then the interrupt
disable/enable instructions (DISI and ENI) can be used to control interrupt routine
execution. At the point in your main program where operations on shared memory
locations are to begin, disable the interrupts. Once all actions affecting the shared
locations are complete, re-enable the interrupts. During the time that interrupts are
disabled, interrupt routines cannot be executed and therefore cannot access shared
memory locations; however, this approach can result in delayed response to interrupt
events.
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Instruction Set
10-115