Timeline for Why is the Digital 0 not 0V in computer systems?
Current License: CC BY-SA 4.0
14 events
| when toggle format | what | by | license | comment | |
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| Dec 30, 2022 at 7:44 | comment | added | Edin Fifić | You forget that "relay logic" used higher voltages and currents (more power required to do the same job) at extremely low speeds and thus was much more immune to noise as compared to modern logic circuits. We could easily achieve the same RELATIVE insensitivity in modern circuits but that would mean GREATLY slowing them down, defeating their purpose. Even in relay circuits we still have the stray currents, voltages and circuit noise, but relays mostly don't notice them. While the output CONTACTS of a relay can achieve 0.00V, the COIL's "logic zero" (turn off point) is usually a FEW VOLTS. | |
| Dec 28, 2022 at 23:59 | comment | added | PStechPaul | Upvoted just to make it +1. This is more of a theoretical or philosophical observation that does not apply to practical usage, but I don't think negative votes are deserved. | |
| Sep 13, 2019 at 14:12 | comment | added | Elliot Alderson | @SolomonSlow The transient behavior is real but it is easily modeled with an ideal circuit. The resistance that controls the behavior of the coil voltage after the contacts open is the resistance of the coil itself (giving you the benefit of the doubt that there are no leakage currents of any kind). It's a parallel RL circuit at that point, which requires infinite time for the inductor current to fall to exactly zero. Even in the practical world, there is some time when the voltage across the coil is non-zero but the relay's contacts become open...a logical '0' with non-zero voltage. | |
| Sep 13, 2019 at 14:04 | comment | added | Solomon Slow | @ElliotAlderson I think that if you put a scope on a real relay coil, you would see the voltage go through zero on its way to a largeish negative value when the contacts open. But, it's unclear to me whether you're talking about a real circuit, or an ideal circuit. Do ideal contacts arc? If not, then the voltage must go to negative infinity. In any case case, after the contacts have opened and the arc is extinguished the resistance in the ideal circuit will be infinite. Not sure what that does to your time constant. | |
| Sep 13, 2019 at 11:51 | comment | added | Elliot Alderson | @SolomonSlow Imagine an energized relay coil. At some instant in time the contacts that provide current to the coil open. The coil is not just a resistance, it is also an inductance. But the current through an inductor cannot stop instantly. There is energy stored in the coil and it will eventually become heat as it passes through the coil's resistance, which means that a voltage appears across the coil. This voltage is proportional to e\$^{-t/\tau}\$ where tau is the time constant created by the inductance and resistance. So, what value of \$t\$ causes the voltage to become exactly 0V? | |
| Sep 13, 2019 at 8:13 | comment | added | KalleMP | @ElliotAlderson No I cannot, I specifically wrote that it was unlikely to exist which means I have no way to prove that it does. However can you prove that such design value has never been required? I didn't think so. Now go and give the new guy an up vote (to get it back to zero) so he does not get demoralised by nitpicking and go away and we loose one more bright (young) mind because of no good reason. | |
| Sep 12, 2019 at 22:41 | comment | added | Elliot Alderson | @KalleMP Can you give an example of where a voltage of 0.000000V is a "required design value for digital communications"? | |
| Sep 12, 2019 at 22:13 | comment | added | KalleMP | +1 because of unfair criticism. A pure 0V can be easily achieved. It can almost be achieved with a relay and simply with access to devices connected to negative supplies and feedback if desired. That it has been used as a required design value for digital communications does seem unlikely though but that should not be reason to down vote this answer. | |
| Sep 12, 2019 at 19:43 | comment | added | Solomon Slow | P.S., The input, also could be driven low (i.e., connected to ground) by the contacts of another relay, but I'm guessing that would be overkill: I'm guessing that the speed of the logic would be limited by the speed with which the relay contacts could move, and not by any electrical properties. | |
| Sep 12, 2019 at 19:38 | comment | added | Solomon Slow | @ElliotAlderson, Imagine a relay coil. One end of the coil is hard-wired to ground (0V). The other end of the coil is an input to a relay-based logic gate. It can either be "high" (pulled up to V+ by the contacts of another relay), or it can be "low" (allowed to float.) Right now, the input is "low" (floating). If you measure the voltage on the input pin with respect to ground, what voltage would you expect to see? If not 0V, what would be the source? and also, where could you use a superconductor in the circuit to change the "low" voltage? | |
| Sep 12, 2019 at 19:20 | review | Low quality posts | |||
| Sep 12, 2019 at 20:10 | |||||
| Sep 12, 2019 at 14:39 | comment | added | Elliot Alderson | Did your relays use superconductors? I don't think so. | |
| Sep 12, 2019 at 13:35 | review | First posts | |||
| Sep 12, 2019 at 23:40 | |||||
| Sep 12, 2019 at 13:33 | history | answered | Leif | CC BY-SA 4.0 |