Timeline for Why did 1970s front panels prefer momentary toggle switches to push buttons?
Current License: CC BY-SA 4.0
7 events
| when toggle format | what | by | license | comment | |
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| Sep 11, 2019 at 3:28 | comment | added | supercat | @CurtJ.Sampson: Even the button is SPDT, which means it's completely bounce-free, no matter how quickly or slowly it's pushed, but I think I'd rather have a momentary toggle since the button is hard on the finger. | |
| Sep 10, 2019 at 2:45 | comment | added | cjs | @supercat That's the 1802 Membership Card? Interesting design to eliminate some of the usual momentary switches. I note that for the one momentary switch that remains, he uses a push button, not a momentary toggle. | |
| Sep 9, 2019 at 22:27 | comment | added | supercat | @CurtJ.Sampson: Lee Hart's 1802-based computer has SPDT switches that go between VDD and VSS, and uses non-inverting buffers with feedback resistors to latch their state when floating. That's a recent design based on components that would have mostly been available in the 1970s (except a 32Kx8 CMOS SRAM); I don't know if/when machines would have used that technique historically. | |
| Sep 3, 2019 at 7:16 | comment | added | cjs | Actually, I should be clear that it's the toggles on the Altair 8800 that are effectively NO switches; if you can provide examples of machines that do use the technique you suggested (using DPST switches to get break-before-make so one can debounce with an SR flip-flop), that would be great! | |
| Sep 3, 2019 at 3:18 | comment | added | cjs | This is exactly the kind of thought I was looking for. However, the toggles are SPST, electrically identical to pushbuttons. These were debounced using a 74123 pulse generator. I've updated the post to mention this, and added a footnote giving these details and a link to the schematic. Also, I'm not sure that there were any "noncritical" momentary contact swiches on the panel. (The address/data switches didn't need debouncing, but they were not momentary contact and could not be NO or NC push buttons.) | |
| Sep 3, 2019 at 2:20 | comment | added | 比尔盖子 | Yes, this is what I was originally thinking in the comment, and later discarded it. After checking the schematics of PDP-8, I saw some versions of front panels use SPDT but other use SPST for the same button, although I did see the flipflop debouncers were used for the SPDT switches, but it seems they just used whatever was best available at the same, so it's probably not a major factor. Still upvoted as the answer is useful. | |
| Sep 2, 2019 at 20:30 | history | answered | Whit3rd | CC BY-SA 4.0 |