Datasheet

SCBA015A
7–27
Fast GTLP Backplanes With the GTLPH1655
Bus-Hold Circuit
If the input characteristics of the LVTTL side (A port) are recorded in small increments, and over
a narrow range of current, the curve shown in Figure 17 results. This curve clearly demonstrates
the effectiveness of the bus-hold circuit.
To change the logic state stored by the bus-hold circuit, a current of about 250 µA must be
overridden.
This circuit is useful when, for example, all drivers on the bus are in a high-impedance state.
Thus, an undefined state can be prevented.
0 V
1 V
2 V
3 V
–0.3 mA –0.2 mA –0.1 mA 0 mA 0.1 mA 0.2 mA 0.3 mA
Input Current
Input Voltage
4 V
Figure 17. Bus-Hold Characteristics at LVTTL Input of SN74GTLPH1655
There is no bus-hold circuit on the GTLP side (B port). A bus-hold circuit on the GTLP side
would defeat the principle of operation of the open-drain outputs, which take on the
high-impedance state to allow the bus to achieve a logic high state (via the pullup resistors).