Siemens CPU 948 User Manual

Effect of SED/SEE The CPU that executes the operation SED xx (disable semaphore)
accesses a specific byte in the coordinator (provided that no other
CPU has access to that byte already). Once a CPU has reserved access,
the other CPUs can no longer access the memory area protected by the
semaphore (numbers 0 to 31). The area is therefore disabled for all
other CPUs.
Make sure that the coordination functions correctly, all CPUs
requiring access to the same area of global memory must use the same
semaphore.
The SEE xx (enable semaphore) operation resets the byte on the
coordinator. The protected memory area is then once again accessible
to the other CPUs. A semaphore can only be enabled by the CPU that
disabled it.
Use of SED/SEE Fig. 3-8 illustrates the basic sequence of coordinated access using a
semaphore.
START
Operation
successful?
Disable semaphore
Access to sema-
phore protected
global memory
Enable semaphore:
No
Yes
End
SED
SEE
Fig. 3-8 Coordination of access to the global memory
Semaphore Operations
CPU 948 Programming Guide
3 - 76 C79000-G8576-C848-04

Siemens CPU 948 Specifications

General

Brand	Siemens
Model	CPU 948
Category	Controller
Language	English