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

Industrial Ethernet overview 247Incorrect wire type (solid/stranded)Patch cords must be made with stranded wire. Solid wire will eventually suffer frommetal fatigue and crack right at the RJ-45 connector, leading to permanent or intermittentopen connection/s. Some RJ-45 plugs, designed for stranded wire, will actually cutthrough the solid conductor during installation, leading to an immediate open connection.This can lead to CRC errors resulting in slow network performance, or can even disable aworkstation permanently.The permanently installed cable between hub and workstation, on the other hand,should not exceed 90 m and must be of the solid variety. Not only is stranded wire moreexpensive for this application, but the capacitance is higher, which may lead to adegradation of performance.Incorrect wire system componentsThe performance of the wire link between a hub and a workstation is not only dependenton the grade of wire used, but also on the associated components such as patch panels,surface mount units (SMUs) and RJ-45 type connectors. A single substandard connectoron a wire link is sufficient to degrade the performance of the entire link.High quality fast and gigabit Ethernet wiring systems use high-grade RJ-45 connectorsthat are visibly different from standard RJ-45 type connectors.Incorrect cable typeCare must be taken to ensure that the existing UTP wiring is of the correct category forthe type of Ethernet being used. For 10BaseT, Cat3 UTP is sufficient, while fast Ethernet(100BaseT) requires Cat5 and gigabit Ethernet requires Cat5e or better. This applies topatch cords as well as the permanently installed (‘infrastructure’) wiring.Most industrial Ethernet systems nowadays are 100BaseX based and hence use Cat5wiring. For such applications, it might be prudent to install screened Cat5 wiring (ScTP)for better noise immunity. ScTP is available with a common foil screen around 4 pairs orwith an individual foil screen around each pair.A common mistake is to use telephone grade patch (‘silver satin’) cable for theconnection between an RJ-45 wall socket (SMU) and the network interface card in acomputer. Telephone patch cables use very thin wires that are untwisted, leading to highsignal loss and large amounts of crosstalk. This will lead to signal errors causingretransmission of lost packets, which will eventually slow the network down.‘Straight’ vs crossover cableA 10BaseT 100BaseTX patch cable consists of 4 wires (two pairs) with an RJ-45connector at each end. The pins used for the TX and RX signals are 1, 2 and 3, 6.Although a typical patch cord has 8 wires (4 pairs), the 4 unused wires are neverthelesscrimped into the connector for mechanical strength. In order to facilitate communicationbetween computer and hub, the TX and RX ports on the hub are reversed, so that the TXon the computer and the RX on the hub are interconnected whilst the TX on the hub isconnected to the RX on the hub. This requires a ‘straight’ interconnection cable with pin1 wired to pin 1, pin 2 wired to pin 2 etc.If the NICs on two computers are to be interconnected without the benefit of a hub, anormal straight cable cannot be used since it will connect TX to TX and RX to RX. Forthis purpose, a crossover cable has to be used in the same way as a ‘null’ modem cable.