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11.92 5 Organization Blocks

5.3.5 Semaphore technique in multiprocessor mode (SES/SEF)

The user has the use of 32 user semaphores; these are used to protect global memory areas

in the communication RAM.

If two or more PLCs use certain global memory areas in the communication RAM in common,

there is a danger that the PLCs might overwrite each other's data or that invalid temporary

states of the data may be read out. For this reason it is necessary to coordinate access to

common memory areas.

The PLCs are coordinated using semaphores and the commands SES and SEF: A PLC can

only access the common memory area if it has successfully set the semaphore (SES) for that

memory area. The semaphore can only be set by one PLC at a time. If a PLC cannot set the

semaphore or if the semaphore has been reset again with the command SEF, the PLC cannot

access the memory area.

All the PLCs concerned must contain a block with the following structure:

yes no

Operation successful?

Access to the

global memory

areas protected by the

semaphore

Reset semaphore: SEF n

Set semaphore: SES n (n=0 to 31)

End

Begin

The commands SES and SEF must be used by all PLCs which need to access synchronisiert

a common memory area. The use of SES and SEF guarantees that a piece of information be-

longing to a PLC can be transferred in or out of a memory area without interruption by another

PLC.

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1) SINUMERIK 880 only

SINUMERIK 840C/880 (PJ)

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Siemens 880 Manuale delle Istruzioni Pagina 33