Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

322 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Figure 12.5
Schematic 10BaseT system
12.2.4 10BaseF
Collisions are detected by the NIC and so an input signal must be retransmitted by the hub
on all output pairs. The electronics in the hub must ensure that the stronger retransmitted
signal does not interfere with the weaker input signal. The effect is known as far end
crosstalk (FEXT), and is handled by special adaptive crosstalk echo cancellation circuits.
The standard has become increasingly popular for new networks, although there are
some disadvantages that should be recognized:
• The cable is not very resistant to electrostatic electrical noise, and may not be
suitable for some industrial environments
• Whilst the cable is inexpensive, there is the additional cost of the associated
wiring hubs to be considered
• The node cable is limited to 100 m
Advantages of the system include:
• Intelligent hubs are available that can determine which spurs from the hub
receive information. This improves on the security of the network – a feature
that has often been lacking in a broadcast, common media network such as
Ethernet
• Flood wiring can be installed in a new building, providing many more wiring
points than are initially needed, but giving great flexibility for future expansion.
When this is done, patch panels – or punch down blocks – are often
installed for even greater flexibility
This standard, like the 10BaseT standard, is based on a star topology using wiring hubs.
It consists of three architectures. These are:
• 10BaseFL
The fiber link segment standard that is basically a 2 km upgrade to the
existing fiber optic inter repeater link (FOIRL) standard. The original FOIRL