IEC 61784-3-3 describes fundamental communication errors that can be identified for applications with functional safety requirements . The description of these communication errors is adjusted to field buses. These errors may however also occur in other communication systems. As explained in Annex C, some of these errors are also used for the assessment of communication services that do not support safety-critical applications.
Due to an error, fault, or interference, not updated messages are accidentally repeated.
In some cases, the lack of response can be detected, and the message repeated with minimal delay and no loss of sequence, in other cases the repetition occurs later and arrives out of sequence with other messages.
Messages may be delayed or advanced beyond their permitted arrival time window. Causes for this behaviour include errors in the transmission medium, congested transmission lines, interference, and applications sending messages in such a manner that communication services are delayed or denied.
Message errors can be recovered in the following ways using scheduled or cyclic scans, for instance, in field buses:
repetition using spare time at the end of the cycle;
treating the message as lost and waiting for the next cycle to receive the next value.
In case of (a), all subsequent messages in that cycle are slightly delayed, while in case (b) only the resent message is delayed.
Cases (a) and (b) are often not classed as an unacceptable deviation from the target end-to-end latency.
Case (c) would be classed as an unacceptable delay for cyclic, distributed automation functions, unless the cycle repetition interval is short enough to ensure that delays between cycles are not significant and that the next cyclic value can be accepted as a replacement for the missed previous value before the survival time expiries (see Clause C.3) .
Due to a fault or interference, a message is inserted that relates to an apparently valid source entity, so a non-safety related message may be received by a safety-related participant, which then treats it as safety related.