Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 232)

Publication 1785-UM012D-EN-P - July 2005
13-14 Programming Considerations
Influencing Priority Scheduling
Use the UID (user interrupt disable) and UIE (user interrupt enable)
instructions to influence user program scheduling. They can be used to
protect important portions of ladder logic that must be executed through to
completion. The UID/UIE instructions are designed to be used in pairs. For
example:
After a UID instruction has executed, interrupts are postponed. The interrupt
program is placed in the
ready
state. After a UIE instruction has executed,
any user programs that are currently in the
ready
state are checked for
priority. If the ready program is of a higher priority than the currently
executing program, the executing program returns to the
ready
status while
the interrupt program begins executing. While the controller is executing
within a UID/UIE zone, the executing program cannot be interrupted except
by a fault routine.
For more information on programming UID or UIE instructions, see the
PLC-5 Programming Software Instruction Set Reference, publication
1785-6.1.
02
O:013 I:012
01
I:012
02
I:012
03
02
O:013 I:012
01
I:012
04
I:012
03
UID
03
O:013 I:012
04
I:012
02
UIE
Program can be
interrupted
Program can be
interrupted
Program cannot be
interrupted