Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 365)

Coordinate Control
Chapter 13
13-13
Figure 13.7
Incremental and Absolute Commands.
20
10
Y
X
Start point
End point
10 35
Absolute command
G90X10.Y20.;
Incremental command
G91X-25.Y10.;
The selection of a unit system (inch or metric) can be done by
programming either G20 for the inch system or G21 for the metric system.
These unit system G codes should be among the first blocks written in a
program.
Both G20 and G21 are modal, and cancel each other. The default unit
system selected by the control at power-up is determined in AMP by the
system installer.
The currently active unit system is usually displayed on the screen for
softkey level 1 in lines 3 or 4 between the [] symbols. If the screen
selected for display of softkey level 1 is the status screen the active system
G code (G21 or G20) will be displayed among the active system G codes.
These functions are affected by the active unit system (inch or metric).
Position commands
Feedrate commands
Axis feed amount for fixed amount feed operation
Unit system for hand pulse generator (HPG)
13.4
Inch/Metric Modes (G20,
G21)