Just about everything I've read on the Web, including on this site, say that unused TTL and CMOS outputs should be left floating. However, this TI article, Designing with Logic, states:

Unused outputs of a device should not be left unconnected (open).

This is stated at the very end of Section 3, at the bottom of page 7.

Unused outputs typo

Why the discrepancy? Is the referred to article wrong?

  • 4
    \$\begingroup\$ Would you believe it is a typo. The article talks about NOT leaving INPUTs unconnected. \$\endgroup\$ Jun 24, 2016 at 16:37
  • \$\begingroup\$ Agreed. Delete the word "not" and the sentence is correct. \$\endgroup\$
    – Transistor
    Jun 24, 2016 at 16:44
  • \$\begingroup\$ @StainlessSteelRat you should make that an answer. \$\endgroup\$
    – DoxyLover
    Jun 24, 2016 at 17:48

1 Answer 1


This is a typo. The not in the Note (as transistor points out) should not be there. Then it makes sense. Be sure to tie off any unused input, but unused outputs are fine.

Line in TI article, Designing with Logic,

Note: Unused outputs of a device should not be left unconnected (open).

The title of the section where this line is referenced is:

Unused Inputs

If you read the section, it refers to not leaving Inputs floating. No where does it discuss outputs. Section discusses the three alternatives for unused inputs:

  • tie to a rail.
  • parallel unused input with a used input.
  • pull-up resistor.

The report goes into great detail on this, but the ultimate reasons for this is to avoid:

  • gates switching randomly.
  • floating input rising beyond maximums.
  • random states.

This applies to all unused inputs to used gates or latches AND unused gates or latches.


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