Siemens SIMATIC S5 Equipment Manual page 38

Operating Instructions
Notes:
— If the peripheral byte O is used as an interrupt source, the address O may not be used in the
P–area of any other 1/0 module (double addressing).
– After start-up, the interrupts in OB20 to OB22 must be deleted. This is accomplished by read-
ing identifier bits.
In all interrupt and time-controlled OBS, when error OBS are used, or in the event
of an automatic or manual restart, the scratchpad flags must always be saved
prior to interrupt processing (see examples for interrupt processing on the next
page) and restored after interrupt processing.
In each of the cases mentioned, a different area of the applied data block must be
used to prevent mutual interference.
This procedure also applies to the use of system date!
With the S5–155U programmable controller the cyclic program (user program is interrupted at
command limits in case of an interrupt (exception: standard function blocks for which the inter-
rupts can be inhibited for a certain period of time (see technical specifications of the function
blocks). Therefore, the scratchpad flags must be saved.
to timed interrupts.
Scratchpad flags are the flags from 200 to 255. These flags are freely available to the user in the
programmable controller.
As these flags are partially used by the standard function blocks, all of these flags must be saved
when a standard function block is interrupted and must be restored again. This can be accom-
plished by transferring the flags to a data block.
The technical specifications for the standard function blocks outline which scratchpad flags (or
system data) are used.
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