Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 765)

Program Interrupt
Chapter 29
29-7
Figure 29.2
Type 2 Interrupt
Programmed Path
Path of Interrupt
Interrupt
occurs
This block is not
executed unless there
are no motion commands
in the interrupt
Return path
Motions due to
Delayed interrupt
M99 M99
Return path
Motions due to
Immediate Action
interrupt
Part program
path before
interrupt
It is possible to alter the number of blocks that the control re-executes in
reverse when returning to the start position of the interrupt. The number of
return blocks is normally 4; however, it may be altered by programming
one of these M codes:
M900 - zero blocks retraced
M901 - one block retraced
M902 - two blocks retraced
M903 - three blocks retraced
M904 - four blocks retraced
These M codes may be programmed in any block in the main program that
is before the interrupt program is executed. They may also be programmed
within an interrupt though the M code will not go into effect until
execution is returned to the main program. If not programmed, the default
is 4 blocks. The default is automatically reset at the end of program
commands (M02 or M30).