Each time a driver switches state, the current transitions from an initial value to
the final value.
3
Figure 7-5 shows simplified examples of received voltages on lines with different
terminations. In each case, what happens when the initial current reaches
the end of the wires depends on what is at the end of the wires. An RS-485
driver has a low output impedance, so the impedance at the source, or driver, is
less than the line??™s characteristic impedance.
If the termination is greater than the characteristic impedance, the signal oscillates,
or rings, before settling to its final level. The same result occurs if a line
has no termination except the receiver.
The extreme case of a termination greater than the characteristic impedance is
when the wires are open at the far end. The open ends present a discontinuity
to the current, which can??™t continue beyond the ends of the wires. The current
has to go somewhere, so it reflects, or turns around and goes back the way it
came. As the current reverses, its magnetic field collapses. The collapse increases
the electrical charge and induces a voltage. The result is that the receiver sees a
higher voltage than what was transmitted.
Chapter 7
118
If the line has a termination but the value is greater than the characteristic
impedance, the effect is similar but less extreme. Some of the initial current
flows in the termination and the rest reflects.
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