2 ms·

It's a good thing that these machines don't even need -5 volts. With just the positive voltages provided, RS-422 still works, including LocalTalk. I think the

• upofadown 2y ago
  I did a bit of research on this because I was confused about why RS-422 needed -5V. Normally the driver is powered off +5 and 0 Volts for RS-422. By increasing the swing down to -5, the Mac port is compatible with RS-232. So a Mac of this era can connect to a modem with a specially wired cable. No level conversion is required and RS-422 receivers are required to work down to -6V so the Mac is producing completely valid RS-422 at the same time. Most RS-232 receivers of that era had a fair amount of gain with a transition point close to 0V. So only a little minus voltage would be required in practice, not the entire -5V. So just as long as the reversed capacitor was not entirely shorting out the rail things would have worked.
  • johnklos 2y ago
    I've wondered how my machines have worked without negative voltages, and this helps. Thanks! These days, a charge pump device costs just a few dollars from Ali-express, so even though things work fine this way for many things, it wouldn't hurt to make them work for most if not all things. When a replacement power supply does provide negative voltage, I try to use it, even if it does mean adapting -12 volts, which is what most ATX power supplies provide, to -5 volts, which is what, it seems, many 68030 Macs want. Funny - I just did an ATX power supply for a Quadra 630, and it seems that most of the 68040 machines were using -12 volts instead of -5 volts (except the Quadra 605, which happened to use the same form factor and power supply as the LC / LC II / LC III).