Designing RS-485 Links and Networks
111
Public Function MaximumLengthOfShortLine _
(ByVal propagationRate As Integer, _
ByVal driverRiseTime As Integer) As Integer
MaximumLengthOfShortLine = driverRiseTime * 1000 / (propagationRate * 24)
End Function
public Int32 MaximumLengthOfShortLine(Int32 propagationRate, Int32 driverRiseTime)
{
Int32 maximumLengthOfShortLineReturn = 0;
maximumLengthOfShortLineReturn =
driverRiseTime * 1000 / ( propagationRate * 24 );
return maximumLengthOfShortLineReturn;
}
If the rise time is unknown, another way of deciding whether a line is long or
short is to compare the shortest expected bit width and the 1-way cable delay.
This method considers two factors: the reflections may bounce back and forth
several times before settling, and the bit rates at the transmitter and receiver
may vary slightly from each other. As a general guideline, if the bit width is 40
or more times greater than the delay, any reflections will have settled by the
time the receiver reads the bits.
$ 8
Two ways to reduce the effect of reflected voltages are to decrease the cable
length and to increase the rise time. There??™s usually not much you can do about
the physical length of a cable required by a particular application, but you can
control the rise time with the choice of drivers.
An interface should be designed to function at the driver??™s minimum and maximum
rise times.
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