Siemens SINUMERIK 840D sl Function Manual page 40

Detailed description
2.2 Functions
2.2.4 Read/write PLC variable
High-speed data channel
For highspeed exchange of information between the PLC and NC, a memory area is
reserved in the communications buffer on these modules (dualport RAM). Variables of any
type (I/O, DB, DW, flags) may be exchanged within this memory area.
The PLC accesses this memory using 'Function Calls' (FC) while the NCK uses '$ variables'.
Organization of memory area
The user's programming engineer (NCK and PLC) is responsible for organizing (structuring)
this memory area.
Every storage position in the memory can be addressed provided that the limit is selected
according to the appropriate data format (i.e., a DWORD for a 4byte limit, a WORD for a 2-
byte limit, etc.).
The memory is accessed via the data type and the position offset within the memory area.
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
$A_DBW
$A_DBD
$A_DBR
Ranges of values
$A_DBB(n)
$A_DBW(n)
$A_DBD(n)
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Data byte (8 bits)
Data word (16 bits)
Data double word (32 bits)
Real data (32 bits)
<= x <= 255
-32768 <= x <= 3276
-2147483648 <= x <= 2147483647
Basic logic functions: Various NC/PLC interface signals and functions (A2)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0