HEIDENHAIN PROFIBUS-DP (DPV2) - Manual (Page 71)

Functionality in the PROFIBUS DPV2 device
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8.7 Preset value
The preset value function enables adaptation of the position value
from the encoder to a known mechanical reference point of the
system. The preset function sets the actual position of the
encoder to zero (= default value) or to the selected preset value. A
preset value can be set more than once and it can be stored to
the non-volatile memory using PROFIdrive parameter 971.
The preset function has an absolute and a relative operating mode
selectable by bit 11 in the Control word (G1_STW). Bit 11 and bit
12 in the Control word controls the preset in the following way.
Normal operating mode: Bit 12=0
In this mode, the encoder will make no change in the output
value.
Preset mode absolute: Bit 11 =0, Bit 12 = 1
In this mode, the encoder reads the current position value and
calculates an internal offset value from the preset value and the
current position value. The position value is then shifted with the
calculated offset value to get a position value equal to the preset
value. No preset will be made if a negative preset value is used
while trying to initiate an absolute preset.
Preset mode relative: Bit 11 =1, Bit 12 = 1
In this mode the position value is shifted by the preset value,
which could be a negative or a positive value set by encoder
parameter 65000 or 65002.