Siemens SINUMERIK 840D sl Function Manual page 501

Control signals
The control signals contained in the following bytes are only active in the machine axes, which
are currently assigned to the drive:
DB31, ... DBB20 to DBB21
Controller enable
The NC/PLC interface signal:
DB31, ... DBX2.1 (controller enable)
is only effective if the machine axis currently has control of the drive:
DB31, ... DBX96.5 == 1 (status setpoint changeover)
Axes without drive control are subject to functional restrictions and are therefore operated in
the follow-up mode. To do this, the control automatically withdraws the controller enable signals
for these machine axes.
Schematic sequence of a setpoint changeover
Initial state: Machine axis AX1 currently controls the drive.
Objective: Machine axis AX2 should assume control.
Special functions
Function Manual, 01/2015, 6FC5397-2BP40-5BA2
S9: Setpoint exchange - 840D sl only
10.3 Interface signals
501