Bus-Holder

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A bus holder (or Bus-Keeper) is a weak latch circuit which holds last value on a tri-state bus. It looks like two inverters connected back to back.

It is used to prevent CMOS gate inputs from getting floating values when they are connected to tri-stated nets. There may arise a case when both the transistors may get turned on, thus shorting the power supply and ground, which may destroy the CMOS gate completely. In such cases, the bus-holder pulls the input to any one of the known logic levels (0 or 1), which is the last value on that net. It is usually placed in parallel with the tri-stated net.