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[FAQ] What is the speed vs. length correlation for isolated RS-485?

What is the speed vs. length correlation for isolated RS-485?

  • There is an inverse relationship between signal rate (speed) and cable length. The exact relationship depends on the resistance and inductance of the cable itself. When building an RS-485 network, the choice of cable can be as important as the transceiver, in order to ensure reliable communication over the necessary distance. Below is a graphical representation of the signal rate to cable length correlation. Region 3 of the figure shows the maximum cable length regardless of signal rate; in this area of the curve, the DC resistance of the cable causes attenuation in the signal and limits the maximum communication distance. Region 2 illustrates the inverse relationship between signal rate and cable length caused by transmission line losses, which will increase with a longer cable. In Region 1, you can neglect transmission line losses; the driver’s rise and fall times determine the main limitation on the maximum data rate. The RS-485 standard recommends a maximum signal rate of 10Mbps; however, with today’s technology, signal rates as high as 50Mbps are available.

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