Designing RS-485 Links and Networksance of an infinite, open line, and is equal to the line’s characteristic impedance.
The value is constant for any length of wire. At frequencies greater than 100
kHz, which make up most of the energy in digital pulses, the characteristic
impedance is mainly resistive, so the value varies little with frequency." .
When a voltage is first applied to a pair of wires, the voltage source doesn’t
know what lies at the end of the pair. The voltage source sees the load as an infi-
nite, open line. The driver’s initial current is a function of its output impedance
and the line’s characteristic impedance. The initial current flows even in a pair
of open wires, where you might naturally assume that no current flows because
the circuit is incomplete.
Shortly after reaching the end of the line, the current settles to a final value
determined by the applied voltage, the termination, and series resistances in the
line. If the initial and final currents vary, the line sees reflected voltages as the
current settles.
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 differ-
ent 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 oscil-
lates, 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.