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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 319<br />

The location of the connection is important to avoid multiple electrical reflections on the<br />

cable, <strong>and</strong> the Thicknet cable is marked every 2.5 meters with a black or brown ring to<br />

indicate where a tap should be placed. Fan out boxes can be used if there are a number of<br />

nodes for connection, allowing a single tap to feed each node as though it was individually<br />

connected. The connection at either end of the AUI cable is made through a 25-pin<br />

D-connector, with a slide latch, often called a DIX connector after the original consortium.<br />

Figure 12.2<br />

AUI cable connectors<br />

There are certain requirements if this cable architecture is used in a network. These<br />

include:<br />

• Segments must be less than 500 meters in length to avoid signal attenuation<br />

problems<br />

• No more than 100 taps on each segment ie not every potential connection<br />

point can support a tap<br />

• Taps must be placed at integer multiples of 2.5 meters<br />

• The cable must be terminated with a 50 ohm terminator at each end<br />

• It must not be bent at a radius exceeding 24.4 cm or 10 inches<br />

• One end of the cable must be earthed<br />

The physical layout of a 10Base5 Ethernet segment is shown in Figure 12.3.<br />

The Thicknet cable was extensively used as a backbone cable until recently but 10BaseT<br />

<strong>and</strong> fiber is now far more popular. Note that when a MAU (tap) <strong>and</strong> AUI cable are used,<br />

the on board transceiver on the NIC is not used. Rather, there is a transceiver in the MAU<br />

<strong>and</strong> this is fed with power from the NIC via the AUI cable. Since the transceiver is remote<br />

from the NIC, the node needs to be aware that the termination can detect collisions if they<br />

occur. This confirmation is performed by a signal quality error (SQE), or heartbeat, test<br />

function in the MAU. The SQE signal is sent from the MAU to the node on detecting a<br />

collision on the bus. However, on completion of every frame transmission by the MAU,<br />

the SQE signal is asserted to ensure that the circuitry remains active, <strong>and</strong> that collisions<br />

can be detected. You should be aware that not all components support SQE test <strong>and</strong><br />

mixing those that do with those that don’t could cause problems. Specifically, if an NIC<br />

was to receive a SQE signal after a frame had been sent, <strong>and</strong> it was not expecting it, the<br />

NIC could think it was seeing a collision. In turn, as you will see later in the manual, the<br />

NIC will then transmit a jam signal.

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