Chapter 4 Deployment of the CPU 821xNET
CPU ADAM 821x
4-2
ADAM 8000 Manual CPU 821x – Rev 1.1
Principles
A network provides a link between different stations that enables them to
communicate with each other.
Network stations consist of PCs, IPCs, H1/TCP/IP adapters, etc.
Network stations are separated by a minimum distance and connected by
means of a network cable. The combination of network stations and the
network cable represent a complete segment.
All the segments of a network form the Ethernet (physics of a network).
In the early days of networking the Triaxial- (yellow cable) or thin Ethernet
cable (Cheapernet) was used as communication medium. This has been
superseded by the twisted-pair network cable due to its immunity to
interference. The CPU 821xNET module has a twisted-pair connector.
The twisted-pair cable consists of 4 cores that are twisted together in pairs.
Due to these twists this system is provides an increased level of immunity
to electrical interference.
Where the coaxial Ethernet networks are based on a bus topology the
twisted-pair network is based on a point-to-point scheme.
The network that may be established by means of this cable has a star
topology. Every station is connected to the star coupler (hub/switch) by
means of a separate cable. The hub/switch provides the interface to the
Ethernet.
The hub is the central element that is required to implement a twisted-pair
Ethernet network.
It is the job of the hub to regenerate and to amplify the signals in both
directions. At the same time it must have the facility to detect and process
segment wide collisions and to relay this information. The hub is not
accessible by means of a separate network address since it is not visible to
the stations on the network.
A hub has provisions to interface to Ethernet or to another hub.
Ethernet supports the principle of random bus accesses: every station on
the network accesses the bus independently as and when required. These
accesses are coordinated by a CSMA/CD (Carrier Sense Multiple Ac-
cess/Collision Detection) scheme: every station “listens” on the bus cable
and receives communication messages that are addressed to it.
Stations will only initiate a transmission when the line is unoccupied. In the
event that two participants should start transmitting simultaneously, they
will detect this and stop transmitting to restart after a random delay time
has expired.
Network
Twisted Pair
Star coupler
(Hub)
Access control
