SN54LS74A: What to do with unused input/output pins of the SN54LS74 D flip-flop chip?

An open bipolar input is read as high, but it floats near the switching threshold, so it can be easily influenced by noise. Therefore, bipolar inputs should be tied high or low (high uses less power).

For TTL family devices, the recommendation is to use a pull-up resistor to protect the input against power supply fluctuations. But for LS family devices, this is no longer necessary.

Tie unused inputs directly to V_CC.

Unused outputs should be left open (not connected).

For a used flip-flop, PRE and CLR must be set to whatever your application requires.
For an unused flip-flop, PRE and CLR can both be high (this is harmless).

Thanks Clemens.

One additional note.  If designing for a high reliability/fault tolerant design especially if designed for space.

Inputs are often biased high or low through a weak pullup or down.   This helps prevent a fault from potentially propagating to power.  Ie, if a short internal to device occurred, the external resistor would limit current.   This is extremely unlikely event, but is still considered best practice.

If you are using the FK package, unused NC pins can be tied to ground, as they are physically not connected to die.  This is often requirement for space designs to not have any floating metal to collect charge.

Regards,

Wade