Siemens SINUMERIK 840D sl Function Manual page 159

The maximum tolerance for position actual value switchover should be set to a lower value
than the gantry warning limit:
MD36500 $MA_ENC_CHANGE_TOL (Max. tolerance for position actual value switchover)
The two position measuring systems must, however, have been referenced beforehand. The
relevant measuring system must be selected before referencing is initiated. The operational
sequence is then the same as that described above.
Channel­specific referencing
Gantry axes can also be referenced by channel with the following interface signal:
DB21, ... DBX1.0 (activate referencing)
The value of the leading axis' machine data are used for the axis sequence for channel-
specific referencing:
MD34110 $MA_REFP_CYCLE_NR (Axis sequence for channel-specific referencing)
After the reference point of the leading axis has been reached, the synchronized axes are
referenced first as described above.
Referencing from part program with G74
The referencing and synchronization process for gantry axes can also be initiated from the
part program by means of command G74. In this case, only the axis name of the leading axis
may be programmed. The operational sequence is analogous to that described for axis-
specific referencing.
Position measuring system with distance­coded reference marks
In order that return traverses do not have to be made over large distances, it is possible to
use a position measuring system with distance-coded reference marks as a sole or second
measuring system for gantry axes. In this way the measuring system is referenced after
traversal of a short path (e.g. 20 mm). The procedure for referencing the gantry axes is the
same as that described for the normal incremental measuring system.
Reference:
/FB1/ Function Manual, Basic Functions, Reference Point Approach (R1)
Absolute encoder
During the course of the synchronization compensatory motion, all axes in the gantry axis
grouping traverse to the reference point value of the leading axis defined in the machine data:
MD34100 $MA_REFP_SET_POS (reference point value/destination point for distance­coded
system)
The absolute encoders and distanced-coded encoders of the leading axis will be set to the
current actual position of the leading axis or to the reference point by the following machine
data:
MD34330 $MA_REFP_STOP_AT_ABS_MARKER (Distance­coded linear measuring system
without destination point)
Special Functions
Function Manual, 09/2011, 6FC5397-2BP40-2BA0
2.3 Referencing and synchronization of gantry axes
G1: Gantry axes
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