Practical Industrial Data Networks:Design, Installation and ...

Industrial Ethernet overview 221At one stage the use of thinnet cable was very popular as a cheap and relatively easyway to setup a network. However, there are disadvantages with this approach as a cablefault can bring the whole system down.10BaseT10BaseT uses AWG24 unshielded twisted pair (UTP) cable for connection to the node.The physical topology of the standard is a star, with nodes connected to a hub. The cablecan be category 3 (Cat3) UTP, although that does not support the faster versions ofEthernet.The node cable (hub to node):• Has a maximum length of 100 meters (328 feet),• Consists of four pairs of which only two pairs are used (one for receive andone for transmit) and• Is connected via RJ-45 plugs.The hub can be considered as a bus internally (any signal input on a given port arereflected as outputs on all other ports), and so the topology is a logical bus topologyalthough it physically resembles a star (hub) topology. The following figure showsschematically how the hub interconnects the 10BaseT nodes.Collisions are detected by the NIC and so the hub must retransmit an input signal on alloutput pairs, so that each NIC can also receive its own transmitted signal. The electronicsin the hub must ensure that the stronger retransmitted signal does not interfere with theweaker input signal. The effect is known as far end cross-talk (FEXT), and it is handledby special adaptive cross-talk echo cancellation circuits.HUBRxTxRxTxRxTxTxRxTxRxTxRxMACDTEMACDTEMACDTEStation AStation BStation CATransmitsFigure 15.410BaseT hub conceptAll others receive