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4.3 Execution of the user program
Easy Book
54
Manual, 03/2014, A5E02486774-AF
The size of the user program, data, and configuration is limited by the available load memory
and work memory in the CPU (Page 15). There is no specific limit to the number of each
individual OB, FC, FB and DB block. However, the total number of blocks is limited to 1024.
4.3.1
Processing the scan cycle in RUN mode
For each scan cycle, the CPU writes the outputs, reads the inputs, executes the user
program, updates communication modules, and responds to user interrupt events and
communication requests. Communication requests are handled periodically throughout the
scan.
These actions (except for user interrupt events) are serviced regularly and in sequential
These actions (except for user interrupt events) are serviced regularly and in sequential
order. User interrupt events that are enabled are serviced according to priority in the order in
which they occur. For interrupt events, the CPU reads the inputs, executes the OB, and then
writes the outputs, using the associated process image partition (PIP), if applicable.
The system guarantees that the scan cycle will be completed in a time period called the
The system guarantees that the scan cycle will be completed in a time period called the
maximum cycle time; otherwise a time error event is generated.
● Each scan cycle begins by retrieving the current values of the digital and analog outputs
● Each scan cycle begins by retrieving the current values of the digital and analog outputs
from the process image and then writing them to the physical outputs of the CPU, SB,
and SM modules configured for automatic I/O update (default configuration). When a
physical output is accessed by an instruction, both the output process image and the
physical output itself are updated.
● The scan cycle continues by reading the current values of the digital and analog inputs
from the CPU, SB, and SMs configured for automatic I/O update (default configuration),
and then writing these values to the process image. When a physical input is accessed
by an instruction, the value of the physical input is accessed by the instruction, but the
input process image is not updated.
● After reading the inputs, the user program is executed from the first instruction through
the end instruction. This includes all the program cycle OBs plus all their associated FCs
and FBs. The program cycle OBs are executed in order according to the OB number with
the lowest OB number executing first.
Communications processing occurs periodically throughout the scan, possibly interrupting
user program execution.
Self-diagnostic checks include periodic checks of the system and the I/O module status
Self-diagnostic checks include periodic checks of the system and the I/O module status
checks.
Interrupts can occur during any part of the scan cycle, and are event-driven. When an event
Interrupts can occur during any part of the scan cycle, and are event-driven. When an event
occurs, the CPU interrupts the scan cycle and calls the OB that was configured to process
that event. After the OB finishes processing the event, the CPU resumes execution of the
user program at the point of interruption.