Designing RS-485 Links and Networksrounds with each round of lower amplitude than previous rounds. Eventually
the current settles to a final value determined mainly by the termination, the
driver’s output resistance, and other series resistances.
If the source’s impedance and termination are less than the characteristic
impedance, the voltage on the line gradually rises to its final value.
The extreme case of a termination less than the characteristic impedance is
when the wires are shorted together at the far end. When the current reaches
the end, there is no load, so there is no voltage drop at all. The entire transmit-
ted voltage has to reflect back to the driver. The electric field collapses and the
magnetic field increases, inducing a current.
If the line has a termination but its value is less than the characteristic imped-
ance, the effect is similar but less extreme. Some of the initial voltage drops
across the termination and the rest reflects. Each time the driver re-reflects a
portion of the voltage, the voltage at the receiver rises until reaching the final
value.
If the wires terminate in a resistance exactly equal to the characteristic imped-
ance, the source of the current sees no discontinuity. Instead, the source sees
something that looks exactly like the infinite line the source had assumed when
it applied the voltage to the line. The initial and final currents are equal, and
after a single 1-way cable delay, the entire transmitted voltage drops across the
resistor with no reflections at all.
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The reflections happen fast. Increasing the physical length of a line increases the
amount of time the reflections last. Each reflection bounces from the receiver to
the driver and back, so each new reflected voltage arrives at the receiver after
two 1-way cable delays. For example, a 10-ft cable might have a cable delay of
15 ns. A series of four reflections would last 0.12 μs. plus the initial 15 ns.
Increase the cable length to 1000 ft, and the same reflections last 12 μs.
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If the reflected voltages are large enough and last long enough, they may have
any of several effects on a line. If the receiver sees a reduced voltage, the
receiver’s input may drop below the threshold for the intended logic level, caus-
ing an error in the received data. If the receiver sees a greater voltage, the input
transistors may saturate, slowing the response. A termination of up to 10%
larger than the characteristic impedance may improve the signal quality by