Siemens Sinumerik 840D sl Programming Manual page 517

Activating ASPLINE, BSPLINE, CSPLINE
If an ASPLINE, BSPLINE or CSPLINE is activated within a curve table definition CTABDEF ...
CTABEND, at least one starting point should be programmed before this spline activation.
Immediate activation after CTABDEF should be avoided, otherwise the spline will depend on
the current axis position before the curve table definition.
Example:
Program code
...
CTABDEF(Y,X,1,0)
X0 Y0
ASPLINE
X=5 Y=10
X10 Y40
...
CTABEND
Repeated use of curve tables
The functional relationship between the leading axis and the following axis calculated using
the curve table will be retained under the selected table number after the end of the part
program and POWER OFF if the table has been saved to the static NC memory (SRAM).
A table created in the dynamic memory (DRAM) will be deleted on POWER ON and may
have to be regenerated.
Once created, the curve table can be applied to any axis combinations of leading and
following axis and is independent of the axes used to create the curve table.
Overwriting curve tables
A curve table is overwritten as soon as its number is used in another table definition.
Exception: A curve table is either active in an axis coupling or locked with CTABLOCK.
Note
No warning is output when curve tables are overwritten.
Curve table definition active?
The $P_CTABDEF system variable can be used at any time in the part program to check
whether a curve table definition is active.
Job planning
Programming Manual, 02/2011, 6FC5398-2BP40-1BA0
Axis couplings
9.2 Curve tables (CTAB)
517