Siemens SINUMERIK 828D Function Manual page 60

A2: Various NC/PLC interface signals and functions
1.3 Functions
Access from NC
To allow the NC to access PLC variables (from a part program) quickly, $ variables are provided in the NCK. The
PLC uses a function call (FC) to read and write $ variables. Data are transferred to and from the NCK
immediately.
$ variables can be accessed (by the NCK) during preprocessing and in synchronized actions.
Data type information is determined by the $ variable data type, the position index is specified as an array index
(in bytes).
The following $ variables are available:
$A_DBB Data byte (8 bits)
$A_DBW Data word (16 bits)
$A_DBD Data double word (32 bits)
$A_DBR Real data (32 bits)
Ranges of values
$A_DBB(n) <= x <= 255
$A_DBW(n) -32768 <= x <= 3276
$A_DBD(n) -2147483648 <= x <= 2147483647
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 1-9 Communications buffer (DPR) for NC/PLC communication
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Function Manual, 09/2011, 6FC5397-0BP40-2BA0
Basic Functions