Siemens SIMATIC S5 Manual page 109

IP 266
Set reference point
Mode 5 allows you to set a reference point without moving the axis. To do so,
select the "Set" function in mode 5. By invoking this function, you can set the
reference point at an arbitrary axis position. The value specified in the machine
data is then assigned to this position.
Tool offsets and zero offsets that existed before this mode was started will still be
taken into account.
Note:
A reference point set with the "Set reference point" function is not
reproducible.
Only when there is positive mechanical coupling can this function be used in drive
systems that are not backlash-free.
The backlash is lost when the reference point is set. In this case, you must
establish a positive mechanical coupling by traversing a distance greater than the
value of the backlash.
EWA 4NEB 812 6057-02
Machine Data, Modes and Traversing Programs
5-37