Datasheet

SCBA015A
7–20
Fast GTLP Backplanes With the GTLPH1655
Comparison Between BTL and GTL
The structure of the two bus concepts (BTL and GTL) is similar. Both operate with
open-collector/open-drain outputs and correct line termination. The most obvious difference is
the definition of the logic voltage levels (Figure 11). The characteristics are listed and compared
in Table 4. For large backplane wiring systems, the BTL circuits have the better characteristics,
whereas the GTL bus features significantly lower power consumption. The target applications,
which were considered when designing each of these bus systems, are apparent: BTL for large
backplane systems, and GTL for smaller buses on a circuit board.
V
OH
= 2.1 V
V
IH
= 1.62 V
V
TH
= 1.55 V
V
IL
= 1.47 V
V
OL
= 1.0 V
V
OH
= 1.2 V
V
IH
= 0.85 V
V
REF
= 0.8 V
V
IL
= 0.75 V
V
OL
= 0.4 V
BTL GTL
Figure 11. Comparison of Logic Voltage Levels of BTL and GTL
Table 4. Comparison of Characteristics of BTL and GTL
CHARACTERISTICS BTL GTL
Capacitance of an I/O pin 5 pF
Not defined,
typically 5 pF to 9 pF
I
OL
100 mA 40 mA
Maximum power consumption of an output driver 100 mW 16 mW
Minimum Z
O
for point-to-point connection 11 Ω 20 Ω
Minimum Z
O
for a bus system 22 Ω 40 Ω
Precharge for withdrawing and reinserting boards during operation Yes No