Siemens sinumerik 840D sl Function Manual page 259

CTABINV
When using the inversion function for the curve tables CTABINV, it must be noted that the
following value mapped to the leading value may not be unique.
Within a curve table, the following value can assume the same value for any number of
master value positions. In order to resolve this ambiguity, the program command CTABINV
requires a further parameter, in addition to the following value, which it uses to select the
'correct' master value. CTABINV always returns the master value that is closest to this
auxiliary parameter. This auxiliary value can, for example, be the master value from the
previous interpolation operation.
Note
Although the auxiliary parameter permits calculation of a unique result for the reversal
function of the curve table, it should be noted that numerical inaccuracies may give rise to
contours, which can cause the reversal function to produce results that deviate from those
that would be obtained in a calculation where the accuracy is unrestricted.
Optional parameters
The functions CTAB, CTABINV, CTABSSV and CTABSEV have optional parameters for the
leading and following axes. If one of these parameters is programmed, the master value and
following value are modified using the scaling factors of the relevant axes.
This is particularly important if axes have been configured with different length units
(inch/metric). If no optional parameters are programmed, the master value and following
value are treated as path positions in the conversion from external to internal representation.
This means that the values are multiplied according to the configured resolution (decimal
places) and the remaining decimal places are truncated.
Special functions: Axis Couplings and ESR (M3)
Function Manual, 11/2006, 6FC5397-2BP10-2BA0
Detailed description
2.2 Curve tables
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