The boundary conditions and assumptions for safety assessments and calculations of residual error rates are listed here.

Generally:

• Number of retries in the underlying standard transmission system :No restrictions
• CRC polynomials used inside the underlying standard transmission system(e.g. Ethernet, TCP, …):No restrictions
• Message storing elements:No restrictions; any number of message storing elements is permitted
• Size of SafetyData within one ResponseSPDU:≤ 1 500 octets

Even for safety functions that do not require manual operator acknowledgment for restart, manual operator acknowledgment is mandatory whenever the SafetyConsumer has detected certain types of errors and indicates this using OperatorAckRequested. Hence, operator acknowledgment is expected to be implemented by the safety application whenever OPC UA Safety is used. For details, see 6.3.4.3 and Clause B.2.