Datasheet

SCBA015A
7–28
Fast GTLP Backplanes With the GTLPH1655
GTLP Output Characteristics
Because the SN74GTLPH1655 has been conceived as an interface between LVTTL partial
systems and a GTLP backplane, the output characteristics of both sides are shown here. The
characteristics for the various logic states of the output stage are shown in a single
voltage-current diagram.
The principle of the GTLP bus is based on open-drain drivers, as shown in Figure 18.
V
TT
V
TT
R
TT
R
TT
To Other Inputs
InputOutput
GTLP Bus
Figure 18. GTLP Bus: An Open-Drain Bus
The device actively drives only the low state on the bus; whereas, for the high state, the required
current flows directly from the termination voltage source, V
TT
. The current is limited only by a
pullup resistor (R
TT
), which usually is of very low resistance. According to the specification, the
pullup resistor must be not less than the minimum value of 22 Ω. A primary purpose of resistor
R
TT
is to provide an optimum termination of the bus to avoid line reflections (see
Transmission-Line Theory in Practice ).
Figure 19 shows that in the low state, the output resistance of the GTLP output stage is in the
range of about 4 Ω.