Chapter 7
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 25Ω/1000 ft. In a 4000-ft link, each wire has 100Ω series resistance.
If a physically long line has two 120Ω termination resistors, a large part of the
signal will drop across the wires, and the receiver will see a much smaller differ-
ential 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 resis-
tance, 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.
The higher power consumption is a result of the line’s lower series resistance. In
Figure 7-6, circuits A and B show how adding a 120Ω terminating resistor
decreases the parallel input impedance from 12k to 119Ω. Assuming 30Ω out-
put impedance for each driver, the current on the line increases from 0.4 to 28
mA.
The higher current also reduces the noise margin. The driver’s output imped-
ance absorbs a larger proportion of the output voltage, reducing the differential
voltage at the receivers. If the output impedance of each driver is 30Ω, o n e t h i r d
of the voltage drops across the drivers’ output impedances, leaving only 3.3V
across the termination. The received differential signal is still 3.1V greater than
the receiver’s input threshold, however. Adding a second termination resistor
exaggerates both of these effects.
One way to conserve power is to disable a driver when it isn’t transmitting. If
the communications path is often idle, disabling drivers that aren’t transmitting