3 ms·

Why include that capacitor at all if it doesn't matter whether it works?

rwmj 2y ago
If you look at the traces you can see the capacitor is right next to the power connector, on the -5V rail (which is not used for much, only for the RS422 serial port). The capacitor will be there to smooth the power supply when the machine is just switched on, or there's a sudden load which causes the voltage to "dip" above -5V. Basically it's like a tiny rechargable battery which sits fully charged most of the time, but can supplement the power on demand. So you can see why it probably didn't matter that this capacitor didn't work: It's only needed for rare occasions. RS-422 is a differential form of RS-232 (https://en.wikipedia.org/wiki/RS-422 https://en.wikipedia.org/wiki/RS-422) so being differential it's fairly robust against changes in load if they affect both wires. And the worst that can happen is you lose a few characters from your external modem. In addition, electrolytics can probably work when reversed like this, at least a little bit. It's not exactly optimal and they might catch fire(!).
- phire 2y ago
  > It's only needed for rare occasions. The two RS-422 ports are actually used quite often on these old Macs for printers, modems and apple talk networking. It was the only communication port, as there was no parallel port. They were backwards compatible with RS-232. So it obviously worked well enough. The backwards cap was measured to reduce the voltage to about -2.4v. I suspect that all it did was reduce the maximum range, which started at a massive 1,200 meters for RS-422 (and a good 10m for RS-232)
_whiteCaps_ 2y ago
Also known as the Madman Muntz theory of Engineering :-) https://en.wikipedia.org/wiki/Muntzing https://en.wikipedia.org/wiki/Muntzing
- qingcharles 2y ago
  I never knew there was a name for this :) When I was a demo coder my artist friend would just haphazardly go through all my assembler code and snip random lines out until it stopped working to improve performance.

rwmj 2y ago

If you look at the traces you can see the capacitor is right next to the power connector, on the -5V rail (which is not used for much, only for the RS422 serial port). The capacitor will be there to smooth the power supply when the machine is just switched on, or there's a sudden load which causes the voltage to "dip" above -5V. Basically it's like a tiny rechargable battery which sits fully charged most of the time, but can supplement the power on demand. So you can see why it probably didn't matter that this capacitor didn't work: It's only needed for rare occasions. RS-422 is a differential form of RS-232 (https://en.wikipedia.org/wiki/RS-422 https://en.wikipedia.org/wiki/RS-422) so being differential it's fairly robust against changes in load if they affect both wires. And the worst that can happen is you lose a few characters from your external modem. In addition, electrolytics can probably work when reversed like this, at least a little bit. It's not exactly optimal and they might catch fire(!).

phire 2y ago
> It's only needed for rare occasions. The two RS-422 ports are actually used quite often on these old Macs for printers, modems and apple talk networking. It was the only communication port, as there was no parallel port. They were backwards compatible with RS-232. So it obviously worked well enough. The backwards cap was measured to reduce the voltage to about -2.4v. I suspect that all it did was reduce the maximum range, which started at a massive 1,200 meters for RS-422 (and a good 10m for RS-232)

_whiteCaps_ 2y ago

Also known as the Madman Muntz theory of Engineering :-) https://en.wikipedia.org/wiki/Muntzing https://en.wikipedia.org/wiki/Muntzing

qingcharles 2y ago
I never knew there was a name for this :) When I was a demo coder my artist friend would just haphazardly go through all my assembler code and snip random lines out until it stopped working to improve performance.