Siemens SINUMERIK Series Function Manual page 269

Assignment of the axes
The assignment of the input and output axes is done via the following system variables:
$AN_CEC_INPUT_NCU
$AN_CEC_OUTPUT_NCU
The system variables become effective only after a restart.
Data backup is always undertaken with machine axis names.
Note
The sag compensation can couple the axis only on one NCU, which can also be traversed
from this NCU either via the part program or via a synchronized action.
These variables are set optionally if the axes (input and output) are not available on the local
NCU. If one uses a channel axis name while programming $AN_CEC_INPUT_AXIS and
$AN_CEC_OUTPUT_AXIS, then the system variables $AN_CEC_INPUT_NCU and
$AN_CEC_OUTPUT_NCU become irrelevant.
The control checks whether the two axes can be interpolated from this NCU, i.e. a program
can traverse the axes on this NCU. The axes can be assigned to different channels. Two
axes belonging to different NCUs can also be compensated. Otherwise the control rejects it
with Alarm 17040.
Both axes of compensation must be interpolated on one NCU, i.e. there may be one or two
part programs that traverse the input and output axes on an NCU.
Extended Functions
Function Manual, 03/2013, 6FC5397-1BP40-3BA1
The following axes are allowed for NCU1 as input or output axes in Configuration
1: NC1_AX1, NC1_AX3, NC1_AX4, NC1_AX5, NC2_AX2, NC2_AX6
The data backup from the NCK always delivers the compensation data from the "machine
axis name" view.
Note
The NCU number is to be programmed before the axis name. A sag compensation
between NC1_AX1 and NC1_AX2 is not possible.
and $AN_CEC_INPUT_AXIS
and $AN_CEC_OUTPUT_AXIS
K3: Compensations
4.4 Interpolatory compensation
269