The reflected current eventually returns to the driver. The driver absorbs part of
the reflection and bounces the rest back, resulting in a reduced voltage at the
receiver. The reflections may continue to bounce back and forth for a few
Figure 7-5: The initial voltages across a parallel termination vary depending on the
difference between the termination and the line??™s characteristic impedance.
Designing RS-485 Links and Networks
119
rounds 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 transmitted
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 impedance,
the effect is similar but less extreme. Some of the initial voltage drops
across the termination and the rest reflects.
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