The SafetyProvider calculates the CRC signature (ResponseSPDU.CRC) and sends it to the SafetyConsumer as part of SPDU. This enables the SafetyConsumer to check the correctness of the SPDU including the SafetyData, Flags, MNR, SafetyConsumerID and SPDU_ID by recalculating the CRC signature (CRC_calc).
[RQ8.19] The generator polynomial 0x F4ACFB13 shall be used for the 32-Bit CRC signature.
[RQ8.20] If SafetyData is longer than one byte (e.g. UInt16, Int16, Float32), it shall be decoded and encoded using big-endian order in which the least significant byte appears last in the incremental memory address stream.
[RQ8.21] The calculation sequence shall begin with the highest memory address (n) of the SafetyData counting back to the lowest memory address (0) and then include also the STrailer beginning with the highest memory address.
Figure 21 shows the calculation sequence of the CRC_SPDU using an example SafetyData with the following fields:
The STrailer and SafetyData have a total length of 34 bytes. The calculation of ResponseSPDU.CRC (SafetyProvider) or CRC_calc (SafetyConsumer) is done in reverse order, i.e. starts at byte 33 and ends at byte 0.
For devices where the SafetyData remains at the same value for a longer period of time, it is a viable optimization to store the calculated CRC over the SafetyData and take – in case the SafetyData hasn’t changed, this stored CRC as start value for the CRC calculation of the STrailer.
Note: On the SafetyConsumer, CRC_calc is calculated using data received in the ResponseSPDU, and not from expected values.