Installation manual

Publication 1747-RM001G-EN-P - November 2008
Understanding Interrupt Routines 11-13
SLC 5/03 and higher processors: If this bit is set or reset by the user
program or communications, it takes effect upon the STI timer
expiration or next end of scan (whichever occurs first).
• STI Executing Bit (S:2/2) - This bit is set when the STI file is being
scanned and cleared when the scan is completed. The bit is also cleared
on power-up and entry into the REM Run mode.
• STI Resolution Selection Bit (S:2/10) - This bit is clear by default. When
clear, this bit selects a 10 ms increment for the STI Setpoint (S:30) value.
When set, this bit selects a 1 ms increment for the STI Setpoint (S:30)
value. To program this feature, use the data monitor function to
set/clear this bit, or address this bit with your ladder program.
This bit is user configurable and takes effect on a REM PROG to REM
RUN mode transition.
• Overflow Bit (S:5/10) - This minor error bit is set whenever the STI
timer expires while the STI routine is executing or disabled while the
pending bit is set. When this occurs, the STI timer continues to operate
at the rate present in word S:30. If the overrun bit becomes set, take the
corrective action your application dictates, then clear the bit.
• STI Lost Bit (Word S:36/9) - This bit is set anytime an STI interrupt
occurs while the STI Pending bit is also set. When set, you are notified
that a STI interrupt has been lost. For example, the interrupt is lost
because a previous interrupt was already pending and waiting execution.
Examine this bit in your user program and take appropriate action if
your application cannot tolerate this condition. Then clear this bit with
your user program in order to prepare for the next possible occurrence
of this error.
Use the following rungs to initialize and measure the amount of time between
two consecutive STI subroutine executions. The 10 µs timer is also available in
the DII interrupt and I/O interrupt. This application example can also be used
for the Event I/O interrupt or the DII interrupt by replacing S:43 with either
S:44 or S:45 respectively.