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

64Practical Industrial Data Networks: Design, Installation and TroubleshootingFigure 4.11Adding a new device to a EIA-485 busThe distance between the end of the shield and the connection in the device should beno more than 10 mm or ½ inch. The end of the wires should be stripped only far enoughto fit all the way into the connector, with no exposed wire outside the connector. The wireshould be twisted tightly before insertion into the screw connector. Often, installers willstrip the shield from the wire and connect the shields together at the bottom of thecabinet. This is incorrect, as there would be from one to two meters of exposed cablefrom the terminal block at the bottom of the cabinet to the device at the top. This exposedcable will invariably receive noise from other devices in the cabinet. The pair of wiresshould be brought right up to the device and stripped as mentioned above.4.2.4.5 ShieldingThe choices of shielding for an EIA-485 installation are:• Braided• Foil (with drain wire)• ArmoredFrom a practical point of view, the noise reduction differences between the first two areminimal. Both the braided and the foil will provide the same level of protection againstcapacitive noise. The third choice, armored cable, has the distinction of protecting againstmagnetic induced noise. Armored cable is much more expensive than the first two and