Rockwell Automation 8520-ARM2 9/Series CNC AMP Reference Manual Documentation Set - Manual (Page 118)

Home Parameters
Chapter 5
5-16
Automatic Homing Sequence (G28) with Distance--coded Markers
The following outlines automatic machine homing (G28) for a DCM axis
that has not been homed previously. Only axes that have their axis words
programmed in the G28 block are homed. For more information, refer to
Automatic Machine Homing (G28).
1. Execute a G28 block, either through the part program or MDI. The
axis moves at a direction and speed defined in AMP by
G28 Direction to Home and G28 Home Speed, respectively. All
feedrate overrides are disabled throughout an automatic homing
sequence.
The axis comes to a stop once the axis crosses three consecutive
markers on the DCM scale.
Important: To determine an absolute position using DCMs, you
must encounter at least three consecutive markers. Thus, if the axis
position will not accommodate this assumption, the axis must be
moved to another position before attempting a homing operation.
2. Once the axis crosses three consecutive markers on the DCM scale,
the axis stops. Here, the control waits until the following error is
within the range specified in the Inposition Band parameter (refer to
page 7-35).
Once the axis is within the inposition band, the control recalculates
the absolute axis position, based on the position after the axis crosses
the three consecutive markers added to the value in Machine Pos. at
DCM Scale 0.
3. The axis moves to the home position as defined by the parameter
Axis Position After Homing at a speed defined by the parameter
Home Speed from Limit Switch.