Technical data

Fieldbus Communication • 169
ETHERNET
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
5-4-3 Rule
A consideration in setting up a tree topology using ETHERNET protocol is
the
5-4-3 rule. One aspect of the ETHERNET protocol requires that a signal sent
out on the network cable must reach every part of the network within a speci-
fied length of time. Each concentrator or repeater that a signal goes through
adds a small amount of time. This leads to the rule that between any two nodes
on the network there can only be a maximum of 5 segments connected through
4 repeators/concentrators. In addition, only 3 of the segments may be popu-
lated (trunk) segments if they are made of coaxial cable. A populated segment
is one that has one or more nodes attached to it. In Figure 5-5, the 5-4-3 rule is
adhered to. The furthest two nodes on the network have 4 segments and 3 re-
peators/concentrators between them.
This rule does not apply to other network protocols or ETHERNET networks
where all fiber optic cabling or a combination of a backbone with UTP cabling
is used. If there is a combination of fiber optic backbone and UTP cabling, the
rule is simply translated to 7-6-5 rule.
Cabling guidelines
"Structured Cabling" specifies general guidelines for network architecture of a
LAN, establishing maximum cable lengths for the grounds area, building and
floor cabling.
The "Structured Cabling" is standardized in EN 50173, ISO 11801 and TIA
568-A. It forms the basis for a future-orientated, application-independent and
cost-effective network infrastructure.
The cabling standards define a domain covering a geographical area of 3 km
and for an office area of up to 1 million square meters with 50 to 50,000 ter-
minals. In addition, they describe recommendations for setting up of a cabling
system.
Specifications may vary depending on the selected topology, the transmission
media and coupler modules used in industrial environments, as well as the use
of components from different manufacturers in a network. Therefore, the
specifications given here are only intended as recommendations.