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Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

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Ethernet <strong>and</strong> TCP/IP networks 321<br />

Figure 12.4<br />

10Base2 ethernet segment<br />

12.2.3 10BaseT<br />

The use of Thinnet cable was, <strong>and</strong> remains, very popular as a cheap <strong>and</strong> relatively easy<br />

way to set up a network. However, there are disadvantages with this approach. A cable<br />

fault can bring the whole system down very quickly. To avoid such a problem, the cable is<br />

often taken to wall connectors with a make–break connector incorporated. The connection<br />

to the node can then be made by ‘fly leads’ of the same cable type. It is important to take<br />

the length of these fly leads into consideration in any calculation on cable length! There is<br />

also provision for remote MAUs in this system, with AUI cables making the node connection,<br />

in a similar manner to the Thicknet connection.<br />

The 10BaseT st<strong>and</strong>ard for Ethernet networks uses AWG24 unshielded twisted pair (UTP)<br />

cable for connection to the node. The physical topology of the st<strong>and</strong>ard is a star, with<br />

nodes connected to a wiring hub, or concentrator. Concentrators can then be connected<br />

to a backbone cable that may be coax or fiber optic. The node cable can be category 3,<br />

although you would be well advised to consider category 5 for all new installations. This<br />

will allow an upgrade path as higher speed networks become more common, <strong>and</strong> given the<br />

small proportion of cable cost to total cabling cost, will be a worthwhile investment. The<br />

node cable has a maximum length of 100 meters; consists of two pairs for receive <strong>and</strong><br />

transmit <strong>and</strong> is connected via RJ45 plugs. The wiring hub can be considered as a local bus<br />

internally, <strong>and</strong> so the topology is still considered as a logical bus topology. Figure 12.5<br />

shows schematically how the 10BaseT nodes are interconnected by the hub.

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