Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 259)

1 Publication 1785-UM012D-EN-P - July 2005
Chapter
17
Using Selectable Timed Interrupts
Using This Chapter
Using a Selectable Timed
Interrupt A selectable timed interrupt (STI) tells the controller to periodically interrupt
program execution (due to elapsed time) to run an STI program once to
completion. Then, the controller resumes executing the original program file
from where it was interrupted. For example, you might want to use an STI to
periodically update analog values for a process control loop or send machine
data to a host at scheduled intervals.
Writing STI Ladder Logic
Follow these guidelines when you write ladder logic for an STI.
• Store the STI program in its own ladder file.
• Make sure that the interrupt interval you specify (in status word S:30) is
longer than the execution time of the STI program. If it is not, an STI
overlap can occur and the controller sets a minor fault bit at S:10/2.
• Note that the controller’s watchdog timer continues to run while the
controller runs an STI program.
For Information About Go to Page
Using a selectable timed interrupt 17-1
Defining a selectable timed interrupt 17-3
Monitoring selectable timed interrupts 17-4
IMPORTANT
If the interrupt occurs during the execution of an
instruction, the controller stops executing the instruction,
scans the interrupt file once to completion, and then
resumes executing the instruction. In effect, STI execution
is transparent to program execution time unless you specify
too short an interval. An interval that is too short can
cause the watchdog timer to time out or cause excessively
long program scans.