Chapter 7
input A more positive than input B. Adding a termination lowers the open-cir-
cuit differential voltage to just a few millivolts. This chapter shows how to add
circuits that replace the internal fail-safe circuits on terminated lines.!
1
Another type of termination used in some systems is the series, or back, termi-
nation. Instead of a parallel resistor across the lines at the end of the cable, the
termination resistor is at the driver, in series with the line (Figure 7-7). The ter-
mination plus the driver’s output impedance equal the line’s characteristic
impedance.
When the output switches, half of the voltage drops across the output imped-
ance and termination. The initial current is half as large as the final current, and
the receiver sees a voltage half as large as the final voltage. The receiver’s high
impedance causes most of the voltage to reflect back to the driver. The driver
and termination, which together equal the characteristic impedance, absorb the
entire reflection. The voltage thus doubles and brings the voltage and current to
their final values after just one reflection.
This type of termination can be useful in full-duplex lines between a single
driver and receiver. The termination uses much less current than a parallel ter-
mination. This termination isn’t recommended for networks with multiple
nodes because the nodes at different locations on the line will see different
reflections.1
3
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If the calculations show that a line is electrically short, you might need no
added termination at all. But on some short lines with fast rise times, the com-
ponents form a resonant circuit that results in ringing voltages when an output
switches. In these cases, a termination can ensure good signal quality at the
receiver.Figure 7-7: A series termination can absorb reflections.