I want to feed an output of the 74LS05 open collector inverter into one of the inputs of the 74LS09 AND gate. The internal schematics in the datasheet shows that there is an internal resistor and 2 diodes between VCC and inputs:

Diagram screenshot

Can I treat the AND gate as having an internal pullup and thus avoid an extra pullup resistor for the 74LS05 or do I still need an external pullup resistor?

  • 3
    \$\begingroup\$ No. It never is safe to assume that any variety of old, bipolar TTL will float up to a solid logical 1 at a floating input. There is no guarantee that the simplified schematic on any datasheet is the actual circuit. \$\endgroup\$
    – AnalogKid
    Oct 23 '20 at 22:25
  • \$\begingroup\$ LS inputs tend to float high, but it is safer with resistor. What kind of signal will it be, how high speeds it must work with? \$\endgroup\$
    – Justme
    Oct 23 '20 at 22:27

A floating TTL input can be considered 1.

Then again, one cannot rely on this. Neither switching speed nor the noise immunity will be the same.

Leaving a TTL input to pull itself is good only for "try and see" and nothing more.


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