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74LS06

The picture above is the SN74LS06 schematic from Texas Instruments. It appears that the input of the SN74LS06 can be connected to an open-drain output.

I purchased the 74LVC06APW from Nexperia. But I don't get any message about design unity.

Are all 74LVC06 and 74LS06 devices compatible? Can I connect the input of the 74LVC06APW to an open-drain output? Or it is better to add a pull-up resistor?

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  • \$\begingroup\$ The input of a 74LS06 will probably appear high when connected to a "High" open-drain or open collector output, but you should use a pull-up resistor to be sure. \$\endgroup\$
    – Peter Bennett
    Aug 21, 2022 at 3:00

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You must connect the open-drain driver to a pull-up resistor to drive the 74LVC06APW input.

The 74LVC06APW will output a tiny current (its input leakage current, IIK) but that will not pull strongly to the upper rail.

A pull-up of 10..47K should be plenty strong enough.

But the actual value will mainly be a function of the load capacitance and the switching frequency.

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  • \$\begingroup\$ To be clear: this is true whether it's open-collector or open-drain. Logic inputs should never be left open-circuit, even if they have some natural pull-up tendency, like TTL does. \$\endgroup\$
    – Tim Williams
    Aug 21, 2022 at 7:38
  • \$\begingroup\$ Nothing's unclear, @TimWilliams. The question and the answer are about a specific circuit, not the general case. \$\endgroup\$
    – TonyM
    Aug 21, 2022 at 9:38

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