Is it possible to leave the unused inputs of the general purpose logic ICs open?
Generally, all unused inputs should be tied to either VCC or GND. (Fig. 1) The reason is that the input impedance of general-purpose logic ICs is extremely high, and if left open, an unintended value may be output due to the influence of the surrounding electric field. In addition, at the midpoint potential, through current flows and current consumption increases, and in some cases, it may lead to destruction of the element.
If the input pin does not have a bus hold function and there is no special description in the data sheet, take the above action.
However, for pins (bus pins, etc.) that have the potential to become outputs like bi-directional bus buffers such as the 245, connect both ends of the input/output to the power supply or GND via pull-up/pull-down resistors. (Fig. 2)
It is recommended that both ends of the buffer be pulled up or down to the same potential to avoid unnecessary current flow. However, leave the input pins with bushold, such as those of the ICs of the TC74VCXHxxx Series, open. Large inrush current due to parasitic capacitance (in the order of a few milliamperes) can be a problem even with typical general-purpose ICs when their power supply is on. In order to improve system reliability against device destruction and other failures, their inputs can be tied to VCC via a pull-up resistor or to GND via a pull-down resistor.
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