Datasheet

SCBA015A
7–14
Fast GTLP Backplanes With the GTLPH1655
This demonstrates one of the basic problems of bus systems. Since the amplitude of the
incident wave depends on the voltage divider between the output resistance of the driver and
the impedance of the line (≈25 Ω), a driver is needed with a particularly low output impedance.
Only then is it possible to switch over a bus line with the incident wave. This is made more
difficult because there are only two bus drivers at the beginning or end of the line. Most drivers
sit in the middle of the bus line and, from there, must effectively drive two lines, one to the left
and one to the right (see Figure 7). In this case, the resulting load impedance for drivers in the
middle of the bus line effectively is halved (≈12.5 Ω).
Taking into account the voltage-divider rule for the incident wave, with TTL-compatible systems,
an output resistance of <10 Ω is needed for the rising edge, and <4 Ω for the falling edge; the
assumption here is that V
OH
= 3.5 V, V
OL
= 0 V. Even the most modern bus-driver families (such
as the ABT family) do not have an output resistance that meets this requirement. For such
applications, Texas Instruments (TI) offers special TTL-compatible circuits, featuring the low
output resistances that are needed: the incident wave switching (IWS) devices from TI,
SN74ABT25xxx, for example, the SN74ABT25245. All other circuits that have the required
low-resistance outputs were developed for new bus systems that are not TTL compatible.
Examples of these new bus systems include BTL and GTLP.
Z
O(Total)
= 12.5 Ω
Z
O
= 25 Ω Z
O
= 25 Ω
Figure 7. Load on a Driver in the Middle of the Bus is 0.5 × Z
O
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