EtherCAT - the Ethernet Fieldbus.
https://www.ethercat.org/en/technology.html
Slave Hardware: EtherCAT Slave Controller with direct I/O
The hardware configuration is stored a in non-volatile memory (e.g. an EEPROM), the Slave Information Interface (SII), which contains information about the basic device features, so that the master can read this at boot-up and operate the device even if the device description file is not available. The EtherCAT Slave Information (ESI) file that comes with the device is XML based and contains the complete description of its network accessible properties, such as process data and their mapping options, the supported mailbox protocols including optional features, as well as the supported modes of synchronization. The Network Configuration Tool uses this information for online and offline configuration of the network.
Slave Hardware: EtherCAT Slave Controller with Host CPU
Various manufacturers offer evaluation kits for implementing slave devices. These kits include slave application software in source code, and they sometimes also include a test master. Using an evaluation kit, a fully functional Master-Slave EtherCAT network can be commissioned in just a few steps. The ETG website contains a Slave Implementation Guide with useful tips and hints on further documentation for implementing slave devices:
ETG.2200 EtherCAT and EtherCAT P Slave Implementation Guide