Siemens SINUMERIK 840D sl Function Manual page 58

A2: Various NC/PLC interface signals and functions
2.3 Functions
Access from NC
System variables are available in the NC for fast access to PLC variables from a part program
or synchronized action. The data is read/written directly by the NC. The data type results from
the identifier of the system variables. The position within the memory area is specified as index
in bytes.
$A_DBB[<index>]
$A_DBW[<index>]
$A_DBD[<index>]
$A_DBR[<index>]
Access from PLC
The PLC uses function calls (FC) to access the memory. The data is read and written
immediately in the DPR with the FC and not just at the beginning of the PLC cycle. Data type
and position in the memory area are transferred as parameters to the FC.
Figure 2-9
Supplementary conditions
● The structuring of the DPR memory area is the sole responsibility of the user. No checks
are made for matching configuration.
● A total of 4096 bytes are available in the input and output directions.
● Single-bit operations are not supported and must be linked back to byte operations by the
user.
● Since the contents of variables are manipulated directly in the communications buffer, the
user must remember that intermediate changes in values occur as a result of multiple
access operations where a variable is evaluated several times or when variables are linked
(i.e. it may be necessary to store values temporarily in local variables or R parameters or
to set up a semaphore).
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System variable
Double word (32 bits)
Floating point (32 bits)
Communications buffer (DPR) for NC/PLC communication
Data type
Byte (8 bits)
Word (16 bits)
Range of values
0 <= x <= 255
-32768 <= x <= 32767
-2147483648 <= x <= 2147483647
±(1.5·10 <= x <= 3.4·10
−45
Function Manual, 01/2015, 6FC5397-0BP40-5BA2
)
38
Basic Functions