Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 433)

5/02 Processor Only
Chapter 31
Understanding I/O Interrupts –
31–3
Interrupt Latency
The interrupt latency (interval between the detection of an interrupt request
from the specialty I/O module and the start of the interrupt subroutine) is 3.7
milliseconds max. for the SLC 5/02 series B processor, and 2.4 milliseconds
max. for the SLC 5/02 series C and later. During the latency period, the
processor is performing operations that cannot be disturbed by the I/O
interrupt function.
Interrupt Priorities
Interrupt priorities are as follows:
1. Fault routine
2. STI subroutine
3. I/O interrupt subroutine (ISR)
An executing interrupt can only be interrupted by an interrupt having higher
priority.
The I/O interrupt cannot interrupt an executing fault routine, an executing
STI subroutine, or another executing I/O interrupt subroutine. If an I/O
interrupt occurs while the fault routine or STI subroutine is executing, the
processor will wait until the higher priority interrupts are scanned to
completion. Then the I/O interrupt subroutine will be scanned.
Note: It is important to understand that the I/O Pending bit associated with
the interrupting slot remains clear during the time that the processor is
waiting for the fault routine or STI subroutine to finish.
If a major fault occurs while executing the I/O interrupt subroutine,
execution will immediately switch to the fault routine. If the fault was
recovered by the fault routine, execution will resume at the point that it left
off in the I/O interrupt subroutine. Otherwise, the fault mode will be entered.
If the STI timer expires while executing the I/O interrupt subroutine,
execution will immediately switch to the STI subroutine. When the STI
subroutine is scanned to completion, execution will resume at the point that it
left off in the I/O interrupt subroutine.
If two or more I/O interrupt requests are detected by the processor at the
same instant, or while waiting for a higher or equal priority interrupt
subroutine to finish, the interrupt subroutine associated with the specialty I/O
module in the lowest slot number will be scanned first. For example, if slot 2
(ISR 20) and slot 3 (ISR 11) request interrupt service at the same instant, the
processor will first scan ISR 20 to completion, then ISR 11 to completion.