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Designing RS-485 Links and Networks

Characteristic impedance is important because a driver initially sees a transmis-

sion line as a load equal to the line’s characteristic impedance. The value deter-

mines how much current flows in a line when a voltage is first applied and an

output switches. When the receiver’s load matches the cable’s characteristic

impedance, the entire transmitted signal drops across the termination and there

is minimal distortion due to reflections as the voltage and current settle to final,

steady-state values.

And this brings us to guideline #2 for RS-485:

Terminate long lines with the line’s characteristic impedance.

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One way to find a cable’s characteristic impedance is to obtain the value from

the cable’s manufacturer. Manufacturers specify characteristic impedance for

products likely to be used as transmission lines. Many circuits use AWG #24

stranded, twisted pair cable, which has a characteristic impedance of 100 to

150 Ω. Manufacturers can determine the value using any of several methods:

• Calculate the value mathematically from the properties of the cable. The cal-
  culation requires knowing the wires’ diameter, the physical length, the dis-
  tance between the wires, and the effective relative permittivity, which varies
  with insulation type.


• Calculate the value from measured inductance and capacitance. Using an
  impedance bridge, measure the line’s capacitance (C) with the far end of the
  cable open, and measure the line’s inductance (L) with the far end shorted.
  The characteristic impedance is. This calculation ignores the line’s
  series and parallel resistance, which have little effect at high frequencies.


• Find the value empirically by applying a step function to the line and vary-
  ing the termination resistor until there are no reflections. A step function is a
  digital pulse with a very short rise time, which ensures that the pulse con-
  tains high frequencies. Viewing the signal requires an oscilloscope with very
  high bandwidth. When the waveform across the termination is identical to
  the transmitted signal, the termination equals the characteristic impedance.

 

1  

Several options are available for terminating digital lines. Table 7-2 summarizes

the options.

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