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 impedance,
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, causing
an error in the received data. If the receiver sees a greater voltage, the input
transistors may saturate, slowing the response.
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