Siemens SINUMERIK 840Di Function Manual page 27

Synchronize actual value (homing)
Once the servo enable has been set, the actual position of the machine axis does not need
to be synchronized again (homing) if the maximum permissible limit frequency of the
measuring system was not exceeded while the machine axis was not in position-control
mode.
Fig. 2-5
DB31, ... DBX2.2 (Delete distance-to-go/Spindle reset (axis-/spindle-specific))
"Delete distance-to-go" is effective in AUTOMATIC and MDA modes only in conjunction with
positioning axes. The positioning axis is decelerated to standstill following the current brake
characteristic. The distance-to-go of the axis is deleted.
Spindle reset
For a detailed description of the spindle reset, please refer to
References:
/FB/ Description of Functions, Basic Machine; Spindles (S1)
Various NC/PLC Interface Signals and Functions (A2)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
Canceling the servo enable when the machine axis is in motion
Detailed Description
2.1 NC/PLC interface signals
2-15