Bradley Smoker PLC-5 Using Selectable Timed Interrupts, Using a Selectable Timed Interrupt

Chapter 17

Using Selectable Timed Interrupts

Using This Chapter

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

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.

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.

Publication 1785-UM012D-EN-P - July 2005