Rockwell Automation 8520-GUM 9/Series CNC Grinder Operation and Programming Manual Documentation Set - Manual (Page 405)

Axis Motion
Chapter 12
12-33
The G30 command is similar to the G28 command, with the main
difference being that the axis or axes move to an alternate home position
instead of machine home. The command format determines whether the
axes return to a second, third, or fourth alternate home position. Any axis
programmed in the G30 block must have been homed prior to G30
execution.
The alternate home positions are defined for each axis in AMP by your
system installer.
To use the G30 command, follow this format:
G30 X__ Z__;
or (second alternate home position)
G30 P2 X__ Z__;
G30 P3 X__ Z__; (third alternate home position)
G30 P4 X__ Z__; (fourth alternate home position)
The axis words in the above block establish the intermediate point in the
same manner as the G28 code described on page 12-29. The axes move to
their intermediate points defined in the G30 block prior to moving to their
alternate home positions. This intermediate point is the same intermediate
point as the one described with the G28 code. When intermediate values
are programmed in a G28 block, they replace G30 intermediate point
values and visa-versa. This intermediate point is used by the G29
automatic return code.
Only those axes included in the G30 block are sent to the alternate home
position.
If an axis included in the G30 block has not been homed, block execution
stops and the following error message appears:
“MACHINE HOME REQUIRED OR G28”
Important: When the control executes a G28 or G30 block, it temporarily
removes any offsets and compensation during the axis move to the
intermediate point. The offsets and/or compensation are automatically
re-activated during the first block containing axis motion following the
G28 or G30 unless that block is a G29 block. If a G29 follows, the offsets
and/or compensation remain deactivated on the way to the intermediate
point and are re-activated when the axis moves from the intermediate point
back to the point indicated in the G29 block.
12.4.5
Move To Alternate Home
(G30)