Datasheet

SCBA015A
7–9
Fast GTLP Backplanes With the GTLPH1655
Z
O
= 100 Ω
100 Ω
l
SN74LS00 SN74LS00
C
L
l = 0 m
(∼10 pF)
l = 1 m
(∼56 pF)
l = 11 m
(∼616 pF)
C
L
= 10 pF C
L
= 56 pF C
L
= 616 pF
500 mV 5 ns 1 V 10 ns
Figure 3. Waveform on a Line Compared to Waveform With a Load Capacitor
Transmission-Line Theory in Practice
With lines of more than a certain length, the behavior of signals must be analyzed using
transmission-line theory. There is a simple rule that applies in this situation:
If the rise time or fall time of a signal is shorter than twice the line propagation delay time,
transmission-line theory must be used.
In practice, transmission-line theory must be used for a bus line with a propagation delay of
25 ns/m and a signal with an edge rise time of 2 ns, from a line length of 4 cm
(2 ns/25 ns/m × 2). Because buses usually are longer than 4 cm, transmission-line theory is a
necessary basis for examining the physical characteristics of bus lines.
With the frequencies and lengths of lines that now are used commonly in bus systems, the
transmission-line theory can be simplified by neglecting any resistive component of the
impedance. Equations 1 and 2 can be used for lossless lines with sufficient accuracy. Table 2
lists typical values for the characteristic properties of point-to-point lines between two
components and bus lines.