Technical data

Priority Program processing levels have a fixed priority. Depending on this
priority, they can interrupt each other or can be nested within each
other.
The warm restart and error levels differ from the basic levels in that
they can always be nested at operation boundaries whenever the
appropriate event occurs. They can be nested both in the basic levels
and within each other. In the event of errors, the last to occur always
has the highest priority.
A basic level on the other hand can be nested in a lower priority
level only at block boundaries unless this default is changed by
writing the appropriate program in DX 0 (see Chapter 7).
Priority of the "basic levels":
CYCLE
TIMED JOB
TIME INT 5 s ascending priority
TIME INT 2 s
.
.
CONTROLLER INT
PROCESS INT
Example:
A process interrupt occurs during the
processing of a time interrupt. Since the
process interrupt has a higher priority, the
processing of the time interrupt level is
interrupted at the next block boundary and the
PROCESS INTERRUPT program processing level is
activated. If, for example, an addressing
error is detected while the process interrupt
is being serviced, the process interrupt is
stopped immediately at the next operation
boundary to activate the ADF level.
Program Processing Levels
CPU 928B Programming Guide
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