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

EIA-485 overview 63+VRinging–VFigure 4.9Ringing on an EIA-485 signal4.2.4.4 Unbalancing the lineUnbalancing the line does not actually induce noise, but it does make the lines moresusceptible to noise. A line that is balanced will have a ballpark balance between thecapacitance and inductance on it. If this balance is disrupted, the lines then becomeaffected by noise more easily. There are a few ways most EIA-485 lines becomeunbalanced:• Using a star topology• Using a ‘tee’ topology• Using unbalanced cable• Damaged transmitter or receiverThere should, ideally, be no stars or tees in the EIA-485-bus system. If another deviceis to be added in the middle, a two-pair cable should be run out and back from the device.The typical EIA-485 system would have a topology that would look something like thefollowing:Figure 4.10A typical EIA-485