74HC being labelled as 74LS?

Question

Bought several tubes of 74LS00. We test the datasheet spec for V_IH 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 V_OUT as V_IN goes from 1.2 to 1.5 V. The datasheet guarantees V_IH 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 V_OUT and 2.55 consistently gives a low V_OUT. 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.

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:

($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.