Event Execution Priorities And Queuing - Siemens SIMATIC S7-1200 Manual

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4.3 Execution of the user program
4.3.3

Event execution priorities and queuing

The CPU processing is controlled by events. An event triggers an interrupt OB to be
executed. You can specify the interrupt OB for an event during the creation of the block,
during the device configuration, or with an ATTACH or DETACH instruction. Some events
happen on a regular basis like the program cycle or cyclic events. Other events happen only
a single time, like the startup event and time delay events. Some events happen when there
is a change triggered by the hardware, such as an edge event on an input point or a high
speed counter event. There are also events like the diagnostic error and time error event
which only happen when there is an error. The event priorities and queues are used to
determine the processing order for the event interrupt OBs.
The program cycle event happens once during each program cycle (or scan). During the
program cycle, the CPU writes the outputs, reads the inputs and executes program cycle
OBs. The program cycle event is required and is always enabled. You may have no program
cycle OBs, or you may have multiple OBs selected for the program cycle event. After the
program cycle event is triggered, the lowest numbered program cycle OB (usually OB 1) is
executed. The other program cycle OBs are executed sequentially (in numerical order) within
the program cycle.
The cyclic interrupt events allow you to configure the execution of an interrupt OB at a
configured scan time. The initial scan time is configured when the OB is created and
selected to be a cyclic interrupt OB. A cyclic event will interrupt the program cycle and
execute the cyclic interrupt OB (the cyclic event is at a higher priority class than the program
cycle event).
Only one cyclic interrupt OB can be attached to a cyclic event.
Each cyclic event can be assigned a phase shift so that the execution of cyclic interrupts with
the same scan time can be offset from one another by the phase shift amount. The default
phase shift is 0. To change the initial phase shift, or to change the initial scan time for a
cyclic event, right click on the cyclic interrupt OB in the project tree, click "Properties", then
click "Cyclic interrupt", and enter the new initial values. You can also query and change the
scan time and the phase shift from your program using the Query cyclic interrupt
(QRY_CINT) and Set cyclic interrupt (SET_CINT) instructions. Scan time and phase shift
values set by the SET_CINT instruction do not persist through a power cycle or a transition
to STOP mode; scan time and phase shift values will return to the initial values following a
power cycle or a transition to STOP. The CPU supports a total of four cyclic and time-delay
interrupt events.
The startup event happens one time on a STOP to RUN transition and causes the startup
OBs to be executed. Multiple OBs can be selected for the startup event. The startup OBs are
executed in numerical order.
The time delay interrupt events allow you to configure the execution of an interrupt OB after
a specified delay time has expired. The delay time is specified with the SRT_DINT
instruction. The time delay events will interrupt the program cycle to execute the time delay
interrupt OB. Only one time delay interrupt OB can be attached to a time delay event. The
CPU supports four time delay events.
The hardware interrupt events are triggered by a change in the hardware, such as a rising or
falling edge on an input point, or a HSC (High Speed Counter) event. There can be one
interrupt OB selected for each hardware interrupt event. The hardware events are enabled in
Device configuration. The OBs are specified for the event in the Device configuration or with
an ATTACH instruction in the user program. The CPU supports several hardware interrupt
events. The exact events are based on the CPU model and the number of input points.
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