Asynchronous Instructions - Siemens Simatic ET200pro Operating Instruction

Basics of program execution

5.3 Asynchronous instructions

Threshold mechanism for time error OB request
The OB parameter "Enable time error" is used to specify whether the time error OB is to be
called at a defined overload for similar events. You can find the OB parameter "Enable time
error" in the properties of the OB in the "Attributes" category.
When you enable the time error OB (check box selected), use the OB parameter "Event
threshold for time error" to specify the number of similar events in the queue as of which the
time error OB is called. If this parameter has the value 1, for example, the CPU enters the
event DW#16#0002:3502 once in the diagnostics buffer and requests the time error OB
when the second event occurs. The CPU suppresses additional diagnostics buffer entries of
the event DW#16#0002:3502 until all events from this source have been post-processed.
In the event of an overload, you therefore have the option of programming a reaction well
before the limit is reached for similar events and thus before the events are discarded.
The following value range applies to the "Event threshold for time error" parameter:
1 ≤ "Event threshold for time error" ≤ "Events to be queued".
5.3 Asynchronous instructions
Difference between synchronous and asynchronous instructions
During program execution a distinction is made between synchronous and asynchronous
instructions.
The "synchronous" and "asynchronous" properties relate to the temporal relationship
between the call and execution of the instruction.
The following applies to synchronous instructions: When the call of a synchronous instruction
is complete, execution of the instruction is also complete.
This is different in the case of asynchronous instructions: When the call of an asynchronous
instruction is complete, execution of the asynchronous instruction is not necessarily
complete yet. This means the execution of an asynchronous instruction can extend over
multiple calls. The CPU processes asynchronous instructions in parallel with the cyclic user
program. Asynchronous instructions generate jobs in the CPU for their processing.
Asynchronous instructions are usually instructions for transferring data (data records for
modules, communication data, diagnostic data, etc.).
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CPU 1516pro-2 PN (6ES7516-2PN00-0AB0)
Operating Instructions, 09/2016, A5E35873416-AA