That's OK, but I hope you have the facts and the knowledge necessary.
Yes. But only a total rookie and a person with ZERO real (by "real" I mean knowledge and education received in school, not through googgling) electronic knowledge would interpret that as "connect EVERY ground and earth together, including low voltage signal GND and GND which supposed to be isolated from everything else. The very idea of "isolated supply" is violated and made totally pointless if that advice is followed literally and everybudy following it literally just proves his/her lack of knowledge.
Nonsense. Opto isolators are NOT filters, even if they can have that as some side effect. The ONLY purpose of optical isolation is to isolate power from other power sources. That is not achieved if your advice is followed because by connecting GND to earth you actually lose isolation.
Now you really have confused things quite a bit by comparing and mixing things and claiming that I said things I never said. Of course Arduino can be made to work, I never said anything else. What I said, or meant is that it is NOT for people with zero electronic knowledge and NOT for people in hurry, who's wife wants that key board and who is spending hours and hours on simple basic problems. Arduino is for electronic experimenters with great interest in electronic and firmware and for people who who know HOW things work in the electronic world. Even so, I doubt anyone really made an Arduino based controller which can compete with simple and cheap drivers or controllers. None of these have anything to do with Mach3 or UCCNC, other that that those software won't run on Mac. Of course, if he wants to use Arduino that's fine, why should I care, but considering his electronic knowledge, he will spend a LOT of time on trying to solve things and will NEVER be happy in the end. Get back here in two-three years time because right now, you have no case. You don't know if I am right or wrong at the moment about this.
Even if he gets it working, he will be unhappy with the speed and the only reason is Arduino. He will NOT be able to run his machine fast enough considering the size of his machine, which is very large, and also considering, like I also said and meant, that it is a nice build, in my opinion it is a shame he went for the Arduino, but yes, it is his decision, and his decision alone, so I really don't care.
Yes and no. Of course, I know how capacitors work and what they do. BUT... there is no need to debounce ANYTHING in a limit switch circuit because the very idea of limit switch is to detect the FIRST change, i.e. the FIRST raising edge of the low to high transition. It doesn't matter if the switch bounces back and forth a thousand times more because the bounce will have ZERO effect since the limit switch triggered signal is already given and detected and the circuit is already broken. The software will (hopefully) not follow the bounces at all, otherwise it is a crappy software. Like I said, I have no idea about how Arduino handles this, but this is surely the case for both Mach3 and UCCNC. Once the limit switch is triggered the software is disabled and movements are stopped, even if the switch bounces back.
Of course, the capacitor will provide a delay and debounce the switch, but that debounce is totally pointless and there are better ways to do it if you'd find that necessary. Remember that we are not talking about a keyboard with push buttons which should only give one signal for every push and allow giving several signals in a row, like when we type on our keyboards. We are talking about LIMIT switch which should trigger as soon as the circuit is broken, not delayed because there is no point in that delay at all.
Attachment 339086
The above image is from the Mach3 manual. There is no capacitor because there is NO NEED for a capacitor in this circuit. Any other implementation is totally wrong, even if it works.
Considering that last line, I am very surprised by your comments above that and before. Also, if that last line is true it must mean that you are at least, or around 75 years old, which I don't believe you are. So, something does not add up in that last line. Even if you are a genius and received your PhD when you were 15 you must be about 65 today (if you have 50 years of experience in electronic design), which I also don't think you are... but never mind.