Siemens SINUMERIK 840Di Function Manual page 759

Effect of activation
The offset for an axis becomes active when the first motion block for this axis is executed
after the offset is activated.
Example of possible chronological sequence:
G0 X100
X150
X200
The "Zero offset external via system frame" is applied immediately.
Channel-specific system frames can be configured using the machine data
MD28082 MM_SYSTEM_FRAME_MASK.
Programming
Setting a new offset via the axis-specific system variables:
$AA_ETRANS[axis]=R
The instruction below reads the axisspecific active offset value:
R =$AA_ETRANS[axis]
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Note
The read value can then differ from the previously set value, if the set value has not yet been
activated.
The read value corresponds to a value set previously, if the most recently set value has not
yet been activated. The system frame for the "Zero offset external" exists only if it has been
configured.
DRF offset
The DRF offset enables you to set an additional incremental work offset for geometry axes
and special axes in the basic coordinate system using a manual pulse encoder.
The DRF offset can be read via axis-specific system variable $AC_DRF[<axis>].
Reference
/FB2/ Description of Functions, Extended Functions; Jog With/Without Handwheel (H1)
Section: DRF offset
Axis Types, Coordinate Systems, Frames (K2)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0
; A new "Zero offset external" is activated by the PLC during this
motion.
; The new "Zero offset external" is applied due to G0 programming at the
end of the block (X200), if no velocity reserve is available (100%).
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Detailed Description
2.3 Coordinate systems
2-29