Siemens Sinumerik 840D sl Programming Manual page 270

Special Motion Commands
4.6 Settable path reference (SPATH, UPATH)
Further information
During polynomial interpolation - and therefore this always involves polynomial interpolation
in a strict sense (POLY), all spline interpolation types (ASPLINE, BSPLINE, CSPLINE) and
linear interpolation with compressor function (COMPON, COMPCURV) - the positions of all
path axes i are specified by the polynomial pi(U). Curve parameter U moves from 0 to 1 within
an NC block, therefore it is standardized.
The axes to which the programmed path feed is to relate can be selected from the path axes
by means of language command FGROUP. However, during polynomial interpolation, an
interpolation with constant velocity on path S of these axes usually means a non constant
change of curve parameter U.
Control behavior for reset and machine/option data
The G code, defined by MD20150 $MC_GCODE_RESET_VALUES[44] (45th G code group)
is effective after a reset. In order to maintain compatibility with existing installations, SPATH is
set as default value.
The initial state for the type of smoothing is defined with
MD20150 $MC_GCODE_RESET_VALUES[9] (10th G code group).
The axis-specific machine data MD33100 $MA_COMPRESS_POS_TOL[<n>] has an
extended significance: It contains the tolerances for the compressor function and for
smoothing with G642.
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Programming Manual, 02/2011, 6FC5398-2BP40-1BA0
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