scroeffie wrote on 2023-06-17, 23:13:
The original 74xxx series ICs were being replaced by 74LSxxx ICs and 74HCxxx/74HCTxxx over time.
Those aren't exactly identical in their inner workings, though.
The original 74xxx was based on TTL (transistor-transistor logic) and used 5v signals.
Edit: Here's the list from edaboard (link below):
TTL (74xx) True TTL
74L Low power
74S Schottky
74H High speed
74LS Low power - Schottky
74AS Advanced - Schottky
74ALS Advanced - Low power - Schottky
74F(AST) Fast - (Advanced - Schottky)
74C CMOS...................check Vcc levels
74HC (U) High speed - CMOS (Unbuffered output)
74HCT High speed - CMOS - TTL inputs
74AHC Advanced - High speed - CMOS
74AHCT Advanced - High speed - CMOS - TTL inputs
74FCT (-A) Fast - CMOS - TTL inputs (speed variations)
74FCT (-T, -AT) Fast - CMOS - TTL inputs (speed variations)
74AC Advanced - CMOS
74ACT Advanced - CMOS - TTL inputs
74FACT AC, ACT (Q) series
74ACQ Advanced - CMOS - Quiet outputs
74ACTQ Advanced - CMOS - TTL inputs - Quiet outputs
The later models are essentially CMOS (3.3v), but 5v compatible.
However, they're not as strong and have different timings (quicker).
That's the simple version of the story.
It could be that the detection logic was made with 74LSxxx in mind and doesn't work as intended with 74HCTxxx.
Speaking ubder correction, though. 🤷
Edit: Here's an explanation:
https://www.edaboard.com/threads/what-is-the- … -and-74hc.9278/
The 74LS are similar to 74, it seems. They use Schottky diodes.
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