Siemens SIMATIC S5 IP 281 Equipment Manual page 81

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Interrupt Interrupts are evaluated at different locations on different programmable
evaluation controllers.
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Remember the following when setting the interrupt points on the S5– 135L.L
The advantage of fast acquisition after each instruction (in practical terms,
reaction to an interrupt is immediate) must be weighed against the
disadvantage that programming the interrupt–controlled program
processing becomes much more complicated to ensure error–free "inser-
tion" of the interrupt program after each STEP5 instruction.
Among other things, this requires that certain instruction sequences for an
interrupt be disabled by interrupts (either with software via STEP5
instruction or with special functions).
Setting "interrupt at block boundaries" in data block DX O is always
recommended when the reaction time permits.
Particularly for interrupts after every instruction, remember that the signal
states or values of temporary data must not be overwritten in the interrupt
program. These values must be saved at the beginning of the interrupt–
controlled program processing, and reloaded again at the end to ensure
that they are again available unchanged to the running program. This par-
ticularly applies to the scratchpad flag area from flag byte FY 200 to FY 255.
PLC
S5–115H
Instruction boundaries
S5–115U
Block boundaries (standard setting) or
instruction boundaries (can be set in data block DX O)
Interrupt Processing
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