Siemens SINUMERIK 840D sl Function Manual page 1040

P3: Basic PLC program for SINUMERIK 840D sl
13.17 Block descriptions
Without additional programming actions, data consistency is only ensured for 1 and 2 byte
access in the NCK and in the PLC. For the 2-byte consistency this is true only for the data type
WORD or INT, but not for the data type BYTE.
In the case of longer data types or transfer of arrays which should be transferred consistently,
a semaphore byte must be programmed in parameter "IVAR2", which can be used by FC21
to determine the validity or consistency of a block. This handling must be supported by the
NC, i.e. in the part program, by writing or deleting the semaphore byte. The semaphore byte
is stored in the same data field as the actual user data.
The semaphore byte is identified by a value between 0 and 4095 in "IVAR2".
The PLC reads and describes the semaphore byte via FC21 in the same call, which should
transfer the user data. The PLC programmer only needs to set up a semaphore variable. For
access from the NC via the parts program, the semaphore feature must be programmed using
individual instructions according to the flow chart shown below. The sequence is different for
reading and writing variables.
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 is set, data are transferred without a semaphore.
Data exchange with semaphore in PLC (schematic of FC21)
Basic structure in the NC:
1040
Function Manual, 01/2015, 6FC5397-0BP40-5BA2
Basic Functions