Influencing priority scheduling -14, Influencing priority scheduling – Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers User 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.

O:013

I:012

O:013

I:012

UID

O:013

I:012

UIE

Program can be
interrupted

Program cannot be
interrupted