Figure 11 shows the structure of a RequestSPDU which originates at the SafetyConsumer and contains a SafetyConsumerID, a MonitoringNumber (MNR), and one byte of (non-safety-related) flags.

NOTE The RequestSPDU does not contain a CRC checksum.

Figure 11 – RequestSPDU

Figure 12 shows the structure of a ResponseSPDU which originates at the SafetyProvider and contains the safety data (1 – 1500 Bytes), an additional 25 Byte safety code (STrailer) as described in the subsequent clauses, and the non-safety related data (see also 6.2.3.4 for details).

Figure 12 – ResponseSPDU

NOTE To avoid spurious trips, the ResponseSPDU is transmitted in an atomic (consistent) way from the OPC UA platform interface of the SafetyProvider to the OPC UA platform interface of the SafetyConsumer. This is the task of the respective OPC UA mapper, see Figure 2.