[phire]

> 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)