10-19
PHYSICAL DESIGN AND SYSTEM DEBUGGING
at the driving device end. The receiver has no termination. The value of the impedance looking
into the driving device (R
driver
+ R
line
= Z
0
) should approximate the impedance of the line as
closely as possible. In this circuit the ringing dampens out when the reflection coefficient goes to
zero. Figure 10-13 illustrates the series termination.
Figure 10-13. Series Termination
One main advantage of series termination is that only logic power dissipation results so that lower
overall power is required. There is one penalty, however, in that the distributed loading along the
transmission line cannot be used because only half of the voltage waveform is travelling down
the line. There is no limit on the number of loads that can be placed at the end of the series ter-
minated connection. However, the drop in voltage across a series terminating resistor limits load-
ing to maximum 10.
Parallel Terminated Lines
Parallel termination is achieved by placing a resistor of an appropriate value between the input of
the loading device and the ground as shown in Figure 10-14. To determine an appropriate value,
the currents required by all inputs and the leakage currents of the drivers are summed. A resistor
should be selected so that its addition to the circuit does not exceed the output capacity of the
weakest driver. For the type of termination shown in Figure 10-14, only high logic levels need to
be calculated.
Figure 10-14. Parallel Termination
Since the input impedance of the device is high compared to the characteristic line impedance,
the resistor and the line function as a single impedance with a magnitude that is defined by the
value of the resistor.
L = g"
Z
0
= 75 Ω
ReceiverDriver R
L
A
B
C
Z
0
= 75 Ω
ReceiverDriver
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