Siemens SINUMERIK 840Di Function Manual page 1140

Detailed description
2.12 Block descriptions
Only individual variables or arrays can be supported directly by the semaphore technique.
Structure transfers must be subdivided into individual jobs. The user must ensure data
consistency of this structure by programming a semaphore system.
If IVAR2 = -1, data are transferred without a semaphore.
Data exchange with semaphore in PLC (schematic of FC 21)
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Power Line Basic PLC Program (P3)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0