A PROFINET IO Device can spontaneously raise a "Diagnostic Alarm" to its Controller in the event of an anomaly (sensor cable break, overheating, short-circuit), without waiting for a read request.
True
A Diagnostic Alarm is an asynchronous telegram emitted by the Device towards the Controller, with no polling required. It is encoded according to the ChannelErrorType (line break, short-circuit, incorrect configuration, etc.), and the Controller (visible to the programmer in TIA Portal) receives the alarm in its diagnostic buffer and can trigger a handling routine. This event-driven mechanism is one of the major advantages compared with polling-based Modbus.
Always programme an OB diagnostic interrupt to react to Diagnostic Alarms; relying purely on cyclic IO values means you will only notice a broken sensor when the process drifts, not the moment it fails.
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