104 • Fieldbus CommunicationETHERNETWAGO-I/O-SYSTEM 750ETHERNET TCP/IP4.1.3.2.1 ETHERNETETHERNET address (MAC-ID)Each WAGO ETHERNET (programmable) fieldbus coupler or controller isprovided from the factory with a unique and internationally unambiguousphysical ETHERNET address, also referred to as MAC-ID (Media AccessControl Identity). This can be used by the network operating system foraddressing on a hardware level.The address has a fixed length of 6 Bytes (48 Bit) and contains the addresstype, the manufacturer’s ID, and the serial number.Examples for the MAC-ID of a WAGO ETHERNET fieldbus coupler(hexadecimal): 00H-30H-DEH-00H-00H-01H.ETHERNET does not allow addressing of different networks.If an ETHERNET network is to be connected to other networks, higher-ranking protocols have to be used.NoteIf you wish to connect one or more data networks, routers have to be used.ETHERNET PacketThe datagrams exchanged on the transmission medium are called“ETHERNET packets” or just “packets”. Transmission is connectionless; i.e.the sender does not receive any feedback from the receiver. The data used ispacked in an address information frame. The following figure shows thestructure of such a packet.Preamble ETHERNET-Header ETHERNET_Data Check sum8 Byte 14 Byte 46-1500 Byte 4 ByteFig. 4-9: ETHERNET-PacketThe preamble serves as a synchronization between the transmitting station andthe receiving station. The ETHERNET header contains the MAC addresses ofthe transmitter and the receiver, and a type field.The type field is used to identify the following protocol by way ofunambiguous coding (e.g., 0800 hex = Internet Protocol).4.1.3.3 Channel access methodIn the ETHERNET Standard, the fieldbus node accesses the bus usingCSMA/CD (Carrier Sense Multiple Access/ Collision Detection).• Carrier Sense: The transmitter senses the bus.• Multiple Access: Several transmitters can access the bus.• Collision Detection: A collision is detected.Each station can send a message once it has established that the transmissionmedium is free. If collisions of data packets occur due to several stationstransmitting simultaneously, CSMA/CD ensures that these are detected andthe data transmission is repeated.
