Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 259)

1 Publication 1785-UM012D-EN-P - July 2005

Chapter

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.

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Rockwell Automation 1785 PLC-5 User Manual Page 259