User guide

A–B
30
Chapter
30–1
Understanding Selectable Timed Interrupts –
SLC 5/02 Processor Only
This chapter applies to the SLC 5/02 processor only. It covers the following
topics:
• STI operation
• STI parameters
• STD and STE instructions
• STS instruction
• INT instruction
The STI (selectable timed interrupt) function can be used with the SLC 5/02
processor only. This function allows you to interrupt the scan of the main
program file automatically, on a periodic basis, in order to scan a specified
subroutine file.
Basic Programming Procedure for the STI Function
To use the STI function with your main program file:
• Create a subroutine file (range is from 3 to 255) and enter the desired
ladder rungs. This is your STI subroutine file.
Creating a subroutine file is discussed in chapter 7.
• Enter the STI subroutine file number in word S:31 of the status file. (See
page 30–4.) A file number of zero disables the STI function.
• Enter the setpoint (the time between successive interrupts) in word S:30
of the status file (see page 30–4). The range is 10 to 2550 milliseconds
(entered in 10ms increments). A setpoint of zero disables the STI
function.
Important: The setpoint value must be a longer time than the execution
time of the STI subroutine file, or a minor error (overrun bit
S:5/10) will occur.
After you download your program and enter the Run mode, the STI begins
operation as follows:
• The STI timer begins timing.
• At timeout, the main program scan is interrupted and the specified STI
subroutine file is scanned; simultaneously, the STI timer is reset.
• When the STI subroutine scan is completed, scanning of the main
program file resumes at the point where it left off.
• The cycle repeats.
STI Overview
Operation