Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

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318 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems 12.2.1 10Base5 systems • 1Base5 Unscreened twisted pair cables, 1 Mbps, twin cable bus • 10Broad36 Cable television (CATV) type cable, 10 Mbps, broadband This is a coaxial cable system and uses the original cable for Ethernet systems – generically called ‘Thicknet’. It is a coaxial cable, of 50 ohm characteristic impedance, and yellow or orange in color. The naming convention for 10Base5: means 10 Mbps; baseband signaling on a cable that will support 500 meter segment lengths. It is difficult to work with, and so cannot normally be taken to the node directly. Instead, it is laid in a cabling tray etc and the transceiver electronics (medium attachment unit, MAU) is installed directly on the cable. From there an intermediate cable, known as an attachment unit interface (AUI) cable is used to connect to the NIC. This cable can be a maximum of 50 meters long, compensating for the lack of flexibility of placement of the segment cable. The AUI cable consists of 5 individually shielded pairs - two each (control and data) for both transmit and receive; plus one for power. Cutting the cable and inserting an N-connector and a coaxial Tee or more commonly by using a ‘bee sting’ or ‘vampire’ tap can make the MAU connection to the cable. This is a mechanical connection that clamps directly over the cable. Electrical connection is made via a probe that connects to the center conductor and sharp teeth which physically puncture the cable sheath to connect to the braid. These hardware components are shown in Figure 12.1. Figure 12.1 10Base5 hardware components

Ethernet and TCP/IP networks 319 The location of the connection is important to avoid multiple electrical reflections on the cable, and the Thicknet cable is marked every 2.5 meters with a black or brown ring to indicate where a tap should be placed. Fan out boxes can be used if there are a number of nodes for connection, allowing a single tap to feed each node as though it was individually connected. The connection at either end of the AUI cable is made through a 25-pin D-connector, with a slide latch, often called a DIX connector after the original consortium. Figure 12.2 AUI cable connectors There are certain requirements if this cable architecture is used in a network. These include: • Segments must be less than 500 meters in length to avoid signal attenuation problems • No more than 100 taps on each segment ie not every potential connection point can support a tap • Taps must be placed at integer multiples of 2.5 meters • The cable must be terminated with a 50 ohm terminator at each end • It must not be bent at a radius exceeding 24.4 cm or 10 inches • One end of the cable must be earthed The physical layout of a 10Base5 Ethernet segment is shown in Figure 12.3. The Thicknet cable was extensively used as a backbone cable until recently but 10BaseT and fiber is now far more popular. Note that when a MAU (tap) and AUI cable are used, the on board transceiver on the NIC is not used. Rather, there is a transceiver in the MAU and this is fed with power from the NIC via the AUI cable. Since the transceiver is remote from the NIC, the node needs to be aware that the termination can detect collisions if they occur. This confirmation is performed by a signal quality error (SQE), or heartbeat, test function in the MAU. The SQE signal is sent from the MAU to the node on detecting a collision on the bus. However, on completion of every frame transmission by the MAU, the SQE signal is asserted to ensure that the circuitry remains active, and that collisions can be detected. You should be aware that not all components support SQE test and mixing those that do with those that don’t could cause problems. Specifically, if an NIC was to receive a SQE signal after a frame had been sent, and it was not expecting it, the NIC could think it was seeing a collision. In turn, as you will see later in the manual, the NIC will then transmit a jam signal.