FSoE: Is data transmitted little-endian or big-endian?

FSoE (failsafe over EtherCAT) always transmits multi-byte data little-endian.

This is implicitly shown in the ETG.5100 S standard, based on the byte order of (for example, the CRC checksum values) - see, for example, FSoE frame structure explained by examples: The CRC fields are transmitted low byte first, high byte second (= little endian).

Source: ETG.5100 S (D) V1.2.0, e.g., Table 3 - General Safety PDU, freely available on request from the EtherCAT Technology Group (ETG)