Designing RS-485 Links and Networks 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.
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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 maxi-
mum rise times. Table 7-1 shows examples of transceivers with different maxi-
mum bit rates and rise times. The rise time should be no more than 20% of the
bit width. Remember that rise time varies with the load.
The minimum rise time determines whether or not a line is long and requires a
termination. As Table 7-1 shows, with a rise time of 3 ns and a propagation rate
of 125 ps/in., a cable just 1 ft long behaves like a long line.
In contrast, the MAX3080 has very slow drivers with a minimum rise time of
667 ns. The slow rise time increases the maximum length of a short line in the