Bought several tubes of 74LS00. We test the datasheet spec for VIH by sweeping one input from 0 V to 5 V while holding the other input at 5 V, and checking the output. Usually we expect to see a gradual transition in VOUT as VIN goes from 1.2 to 1.5 V. The datasheet guarantees VIH at 2 V minimum, meaning 2 V or higher should be treated as a logic high.

Instead we're seeing an abrupt transition at 2.5 V. 2.45 V consistently gives a high VOUT and 2.55 consistently gives a low VOUT. In other words, it's behaving exactly like a 74HC00. The output behavior under pull-down loads is also closer to what we typically see as 74HC00.

The chip is clearly labelled as 74LS00 but the logo is obscure. Is it possible someone is selling CMOS chips as low-power Schottky? Has anyone else seen this?

You might ask why we care and why we're buying ancient tech. They're for a lab where engineering students are asked to compare the measured performance of CMOS vs low-power Schottky, by rigorously following the datasheet test conditions. The whole exercise fails if the LS is really HC.

  • 2
    \$\begingroup\$ Where did you purchase these chips from? \$\endgroup\$
    – vir
    Jan 26 at 21:13
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    \$\begingroup\$ Depending on where you bought them from, it's totally possible you got screwed. Is there a CoC (certificate of conformity) that goes along with your part order? Anyway, Texas Instruments still sells SN74LS00N directly on their website. I suggest you get the parts directly from them going forward. \$\endgroup\$
    – Ste Kulov
    Jan 26 at 21:21
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    \$\begingroup\$ I have never bought ICs from ebay, AliExpress, Banggood or Amazon. There are entire cities over there where sellers buy scrap junk, relabel it and sell it. Many places make counterfeit parts. All my genuine ICs have very clear well-known logos. They all work perfectly. \$\endgroup\$
    – Audioguru
    Jan 26 at 21:46
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    \$\begingroup\$ The power consumption should tell you. \$\endgroup\$ Jan 26 at 21:55
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    \$\begingroup\$ You can tell them apart by measuring current from unconnected input to GND with a multimeter when the chip is powered. 74LS will have a current of around 0.1 mA, while 74HC will show zero current. ("Input low current" in datasheet) \$\endgroup\$
    – jpa
    Jan 27 at 8:03

1 Answer 1


Yes, if you buy from dodgy sources, remarked parts that are sort-of compatible is one of a number of possibilities (including authentic working counterfeits, counterfeits that don't meet specifications, non-functional parts marked as what was sold, used parts (often straightened and cleaned) sold as new).

On Chinese websites the latter may be indicated as 二手 = (literally) second hand and may be a useful alternative if the originals are unobtanium and you know what you are buying. Here's one from a Taiwan website with an (obsolete) Hitachi HD74LS00:

enter image description here

($60 NTD is pretty expensive for that part, BTW, about $2 USD).

In any case you can reliably get DIP 74LS00 from inventory at US distributors (eg. Digikey) or non-dodgy places like lcsc.com, the latter having both TI and Asian brands such as Xinluda.

BTW, in the case of Xinluda there's no guarantee that the maker didn't decide that a 74HCT00 die was close enough- if it will meet all the specifications on their datasheet. I don't see a spec for minimum input current or minimum supply current, for example.

  • \$\begingroup\$ Spehro, thanks for your interesting and useful answer. The LS spec that HC parts "should" always fail is the input threshold voltage when VDD=5V. Vin = 2.0 volts should be high for TTL logic but low for CMOS. \$\endgroup\$
    – Eeyn
    Jan 27 at 15:25
  • \$\begingroup\$ @Eeyn that’s why I mentioned HCT which has TTL input thresholds. \$\endgroup\$ Jan 27 at 18:41

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