Asynchronous Instructions - Siemens SIMATIC ET 200SP System Manual

Basics of program execution

7.3 Asynchronous instructions

Threshold mechanism for time error OB request
The cyclic interrupt OB parameter "Enable time error" is used to specify whether the time
error interrupt should be called when a specific overload level is reached for similar events.
You can find the OB parameter "Enable time error" in the properties of the OB in the
"Attributes" category.
If you activate 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 from which the
user program is to call the time error interrupt. 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".
7.3 Asynchronous instructions
Difference between synchronous/asynchronous instructions
In program processing, a differentiation 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 ended, the execution is also ended.
This is different in the case of asynchronous instructions: When the call of an asynchronous
instruction is ended, the execution of the asynchronous instruction is not necessarily ended
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 occupy resources in the CPU while they are being processed.
Asynchronous instructions are usually instructions for transferring data (data records for
modules, communication data, diagnostics data).
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Distributed I/O system
System Manual, 12/2015, A5E03576849-AF