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Processing the User Program
Reactions to interrupts
and errors
3.4.1
Definition of Terms used in
Program Execution
Cycle time
3 - 12
To allow you to specify the reactions to interrupts or errors, special
organization blocks (OB 2 to OB 18 for interrupt servicing, OB 19
and OB 23 to OB 34 for reactions to errors) are available on the
CPU 948. You can store an appropriate STEP 5 program in these blocks.
When interrupts or errors are to be processed, the system program
activates the corresponding organization block during cyclic
processing. This means that the cyclic processing is interrupted to
service an interrupt or to react to an error. The nesting of the
organization blocks has a fixed priority (for further information, refer
to Chapters 4 and 5).
In addition to the organization blocks, you can also influence the
reaction of the CPU to interrupt servicing by assigning parameters in
data block DX 0.
Organization blocks OB 1 to OB 39 can be called by the system
program as soon as they are loaded in the program memory (also
during operation).
If the OBs are not loaded, there is either no reaction from the CPU or
(in the event of errors) it goes to the stop mode (refer also to Section
5.4).
You can also load data block DX 0 into the program memory during
operation like the organization blocks. It is, however, only effective
after the next COLD RESTART. If DX 0 is not loaded, the standard
settings apply (refer to Chapter 7).
The cycle begins when the cycle monitoring time is triggered and ends
with the next trigger. The time that the CPU requires to execute the
program between two triggers is called the cycle time. The cycle time
consists of the runtime of the system program and the runtime of the
user program.
The cycle time therefore includes the following:
• • the time required to process the cyclic program (system and user
program),
• • the time required to process interrupts (e.g. time-controlled
interrupt),
• • the time required to process interruptions (errors).
CPU 948 Programming Guide
C79000-G8576-C848-04
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