Datasheet
SCBA015A
7–15
Fast GTLP Backplanes With the GTLPH1655
End of the Line: The Reflected Wave
As explained in
Transmission-Line Theory in Practice
, a voltage wave is reflected at the end of a
line, and this reflected wave moves back to the beginning of the line. The amplitude of the
reflected wave is determined by the amplitude of the incident wave and the reflection factor
(Equation 5). This reflection factor is determined by the line impedance and the termination
resistance (Equation 4). Therefore, the termination resistance has a major influence on the
waveform of a bus line.
For the case of no termination resistance at the end of the line (R
TT
= ∞), as shown in Figure 8a,
the reflection factor is ρ = 1. The amplitude of the reflected wave is, therefore, exactly the same
as the amplitude of the incident wave. In practice, this means that a low-resistance driver that
generates an incident wave of 3 V, generates a reflected wave that also has an amplitude of 3 V.
This results in an overshoot at the end of the line of 6 V (V
incident
+ V
reflected
= 3 V + 3 V). The
worst case would be a very low-resistance CMOS driver with an incident wave of 5 V, which
would result in an overshoot of 10 V at the end of the line.
If the value of the termination resistance is assumed to be exactly the same as that of the line
impedance, a reflection factor of ρ = 0 (Figure 8b) results. In this case, no reflection of the
arriving wave occurs; thus, it is an ideal line termination. However, this method cannot be used
with TTL and CMOS-compatible bus systems, because the impedance of the line would make it
necessary to have a termination resistor of 25 Ω. With bidirectional lines, it would be necessary
to connect this termination resistor at both ends, and each driver then would have to drive a load
of 12.5 Ω. The maximum current through these resistors would be 280 mA (3.5 V/12.5 Ω) per
line. Because, in practice, a bus often has more than 100 lines, the maximum total current of the
bus termination would be >28 A. For this reason, with TTL systems, one operates with other
terminating networks (Figure 8c to 8f) and, in such cases, accepts a mismatch (R
TT
> Z
O
).