Publication 1747-RM001G-EN-P - November 2008
11-22 Understanding Interrupt Routines
Interrupt Priorities
Interrupt priorities for the SLC 5/03 and higher processors are:
1. User fault routine.
2. Discrete Input Interrupt (DII).
3. STI Subroutine.
4. I/O Interrupt Subroutine.
An executing interrupt subroutine can only be interrupted by the fault routine.
Status File Data Saved
Data in the following words is saved on entry to the DII subroutine and
re-written upon exiting the DII subroutine.
• S:0 Arithmetic flags
• S:13 and S:14 Math register
• S:24 Index register
Reconfigurability
You can reconfigure the DII entirely or in part, depending on the particular
parameter(s) you choose. You can reconfigure some of the parameters simply
by writing the new value over the old value. Other values require you to set the
reconfiguration bit in addition to writing the new value. The DII is
non-retentive and always reconfigures itself upon entry into the REM Run
mode. Refer to "DII Parameters" for details on reconfiguring each parameter.
Example
The DII can be programmed to count items on a high-speed conveyer. Each
time 100 items pass a photo-switch, the DII subroutine is executed. The DII
subroutine then uses Immediate I/O instructions to package the products.
TIP
Under certain conditions, though, it is possible for a lower
priority task to run during the DII execution.
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