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Aug 23, 2022 at 5:03 comment added Justme @Jens If the circuitry pulls the input to 1.58V, that is at about the threshold of the transition, as 2.0V is the minimum legal voltage for TTL logic high. CMOS inputs draw no current so the programmable roughly 40k pull-up can pull the voltage to supply voltage, as generally CMOS inputs require about 70% of the supply voltage to be mininum legal logic high voltage.
Aug 23, 2022 at 3:20 comment added Mark Ransom When is the last time you worked with a chip that was actual TTL? I hope you're not suggesting that's the technology behind Arduino.
Aug 22, 2022 at 19:49 comment added PStechPaul Inputs with pull-ups also can be driven by open-collector (NPN) or open-drain (NMOS) devices, which can be wire-ORed together.
Aug 22, 2022 at 18:22 comment added Jens @Justme Thanks for the hint. I was curious about the details and took an original SN7408 from 1974. I measure between 0.77 and 0.84 uA input current providing + 5V and 1.01 mA into GND. The open input voltage is 1.58 V, not really a clear "high" I admit, but this behaves in many aspects like an input with pull-up resistor. It needs about 500 uA to GND to see a low, so I assume this is less susceptible to noise than a modern MCU input with internal pull-up.
Aug 22, 2022 at 17:02 comment added Justme TTL chips technically don't have an internal pull-up resistor, and are not intended to be used without external pull-up. Sure, the inputs tend to float high due to internal structures, and may work even without external resistor. But as TTL input will sink and source current anyway, so it does not compare to a CMOS input anyway.
Aug 22, 2022 at 16:36 history answered Jens CC BY-SA 4.0