|What should I do with the UNUSED INPUTS?
TTL devices normally default to a logic ONE if the input is
allowed to FLOAT.
However, depending on many factors, these inputs can act as an antenna,
and pick up NOISE which can cause mischief to the circuit. The worst case
is, the device will act as an uncontrolled intermittent OSCILLATOR.
--This is BAD!
ALL unused inputs of TTL logic devices IDEALLY should be returned to
either a logic ONE or logic ZERO. Certain devices--74XX and earlier 74LS--the
logic ONE (Vcc) should be a ~ 1k resistor to Vcc; or alternatively, connected
directly to ground (ZERO). Most/All other logic devices, the resistor is
In the case of CMOS devices, there is NO input DEFAULT; CMOS
input impedance is on the order of thousands of MEGOHMS and will
cause EXTREME FAILURES if allowed to FLOAT.
To make this point: if you put your finger or a scope probe on or NEAR
an un-terminated CMOS input, it can cause a logic CHANGE!! If allowed
to float, some CMOS devices can oscillate so fast, that it exceeds the
device power rating and will literally destroy itself.
Unused Outputs: tri-state, and open collector devices, etc.,
DO NOT require any attention.
NOTE: In the case of the tri-state control pin, it should be treated
as an INPUT.
Notes  Tri-state: Zero, One, Open.