A termination of up to 10%
larger than the characteristic impedance may improve the signal quality by
Chapter 7
120
increasing the initial received voltage. In extreme cases, a mismatch can cause
reflections so large that they damage components.
The termination rarely matches the characteristic impedance exactly. But a
value that??™s reasonably close reduces the amplitude of the reflections and
improves signal quality overall.
6 5
A line??™s series resistance has little effect on the characteristic impedance at high
frequencies, but the series resistance can become significant for other reasons
when the wires are very long. The resistance of stranded AWG #24 wire is
about 25?„¦/1000 ft. In a 4000-ft link, each wire has 100?„¦ series resistance.
If a physically long line has two 120?„¦ termination resistors, a large part of the
signal will drop across the wires, and the receiver will see a much smaller differential
voltage. However, if the signals have the minimum 1.5V difference at the
driver, only a fraction of the signal needs to make it to the receiver to enable
detecting the minimum required 0.2V difference. To decrease the series resistance,
use wire with a lower AWG value (and thus a larger diameter).
%
Adding a termination is a trade off. Besides reducing reflections, terminating an
RS-485 line has negative effects, including increased power consumption, lower
noise margin, and overriding the receiver??™s internal fail-safe circuits.
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