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Publication LOGIX-AP008B-EN-P - June 2008 39

Logix Features that May Not be Familiar to S7 Users Chapter 2

The number of periodic OBs available depends on the type of controller.

Lower priority number represents higher interrupt priority (priority selection is

only available with S7 400 controllers). Execution (ms) is the execution period

for the OB. Phase offset allows phasing the triggering of periodic interrupts

relative to each another. The process image partition selection allows the I/O

image table to be partitioned and that partition only updated when the

interrupt occurs (this feature is available with S7 400 controllers only). Default

is the full table. In Logix, see the task I/O update selection and IOT

commands.

The content of a periodic interrupt OB typically resembles the content of

OB1. It will consist of calls to functions and function blocks that are to be

executed at the periodicity of the OB.

These OBs resemble periodic tasks in Logix. In S7 terminology OB30 –

OB38 are called Cyclic Interrupt OBs.

OB40 – OB47 Hardware Interrupt OBs

These OBs may be configured to trigger on an input event. Their priority may

also be configured.

These are event tasks in Logix. In S7 terminology OB40 – OB47 are called

Hardware Interrupts.

For example, the most simple hardware event that could be handled by a

Hardware Interrupt OB (or Event Task) is a change of state of a digital input.

A Hardware Interrupt (or Event Task) would guarantee a very fast response to

the change.

Event tasks are more flexible than Hardware Interrupt OBs, with triggers not

only from I/O, but also from network events, program instructions and

motion events.

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Siemens 7 Manuel d'utilisateur Page 39