Semaphore Technique - Siemens SINUMERIK 880 SW 6 Planning Manual

16 PLC/PLC Interface (DB 50 ... DB 53, DB 59)

16.4.1 Semaphore technique

16.4.1 Semaphore technique
The user has 32 user semaphores at his disposal; they have the function of protecting the me-
mory areas in the link RAM DB 59.
If two PLCs both use certain global memory areas in the link RAM, there is a danger of one
PLC overwriting another's data or of invalid interim data states being read out. Access of the
PLCs to the common memory areas must therefore be coordinated.
The individual PLCs can be coordinated with the semaphores and the commands SED and
SEE: only after the agreed semaphore SED has been set does one of the PLCs (the one
which set the semaphore) access the common memory area. A semaphore can be set by only
one PLC in each case. If a PLC fails to set the semaphore, no access is possible. Similarly, a
PLC must forego further access once it has enabled the semaphore again (SEE).
Both PLCs must contain a function block with the following program structure:
Yes
Access to the global
memory area
protected by the
semaphore
Enable semaphore: SEE xx
PLC I
FB 200
SED 20
JZ End
Program part data
processing
: SEE 20
End : BE
16–10
Begin
Set semaphore: SED xx
Operation successful?
End
DB 59
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Siemens AG 1991 All Rights Reserved
Scan JZ
No
PLC II
FB 201
SED 20
JZ End
Program part data
processing
: SEE 20
End : BE
6ZB5 410-0HE02
SINUMERIK 880 (PJ)
01.93