Siemens SINUMERIK 840D sl Function Manual page 41

Access from PLC
The PLC uses function calls (FC) to access the memory. These FCs ensure that data are
read and written in the DPR immediately, i.e., not just at the beginning of the PLC cycle. FCs
receive data type information and the position offset as parameters.
Figure 2-9
Supplementary conditions
• The user's programming engineer (NCK and PLC) is responsible for organizing the DPR
• A total of 1024 bytes are available in the input and output directions.
• Singlebit operations are not supported and must be linked back to byte operations by the
• Since the contents of variables are manipulated directly in the communications buffer, the
• The user's programming engineer is responsible for coordinating access operations to the
• Data consistency can be guaranteed only for access operations up to 16 bits (byte and
• The PLC stores data in 'Little Endian' format in the DPR.
• Values transferred with $A_DBR are subject to data conversion and hence to loss of
Basic logic functions: Various NC/PLC interface signals and functions (A2)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
Communications buffer (DPR) for NC/PLC communication
memory area. No checks are made for inconsistencies in the configuration.
user (programming engineer).
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).
communications buffer from different channels.
word). The user's programming engineer is responsible for ensuring consistent
transmission of 32bit variables (double and real). A simple semaphore mechanism is
provided in the PLC for this purpose.
accuracy. The data format for floatingpoint numbers is DOUBLE (64 bits) on the NCK, but
only FLOAT (32 bits) on the PLC. The format used for storage in the dualport RAM is
FLOAT. Conversion takes place respectively before/after storage in the dualport RAM.
Detailed description
2.2 Functions
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