Technical data

178 • Fieldbus Communication
ETHERNET
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
4.1.3.2.1.1 Channel access method
In the ETHERNET Standard, the fieldbus node accesses the bus using
CSMA/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 transmission
medium is free. If collisions of data packets occur due to several stations
transmitting simultaneously, CSMA/CD ensures that these are detected and
the data transmission is repeated.
However, this does not make data transmission reliable enough for industrial
requirements. To ensure that communication and data transmission via
ETHERNET is reliable, various communication protocols are required.
4.1.3.2.2 IP-Protocol
The Internet protocol divides datagrams into segments and is responsible for
their transmission from one network subscriber to another. The stations in-
volved may be connected to the same network or to different physical net-
works which are linked together by routers.
Routers are able to select various paths (network transmission paths) through
connected networks, and bypass congestion and individual network failures.
However, as individual paths may be selected which are shorter than other
paths, datagrams may overtake each other, causing the sequence of the data
packets to be incorrect.
Therefore, it is necessary to use a higher-level protocol, for example, TCP to
guarantee correct transmission.
IP addresses
To allow communication over the network each fieldbus node requires a 32 bit
Internet address (IP address).
Attention
Internet addresses have to be unique throughout the entire interconnected
networks.
As shown below there are various address classes with net identification (net
ID) and subscriber identification (subscriber ID) of varying lengths. The net
ID defines the network in which the subscriber is located. The subscriber ID
identifies a particular subscriber within this network.
Networks are divided into various network classes for addressing purposes: