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PHYSICAL DESIGN AND DEBUGGING
-t>
Zo
=750
A
"NV'v
B
(~
)
C
{>-
•
•
RL
Driver
L=9'~
Receiver
231732i11-13
Figure
11
-13. Series Termination
One main advantage
of
series termination
is
that only logic power dissipation results
so
. that lower overall power
is
required than with other techniques. 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 terminated connec-
tion. However, the drop in voltage across the series terminating resistor limits loading to
a maximum of
10
loads.
11.4.2.1.3 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 11-14. To determine an
appropriate value, the currents required
by
all inputs and the leakage currents of the
drivers should be summed. A resistor should
be
selected so that its addition to the circuit
does not exceed the output capacity
of
the weakest driver.
ZO=750
Driver'
231732i11-14
Figure 11-14. Parallel Termination
11-18