Project for someone...

[exxos]

Idea is to break the bus pull up resistors from 5V and wire in a 3.3V regulator, and connect the pullups to the 3.3V. Technically the bus will run at 3.3V and logic 1 being 2V generally, the H4 *should* still work... but needs trying...

This might ultimately allow some 3.3V chips on the bus without IO converters...

[derkom]

Would the whole bus have to be converted to 3.3V to test this, or would it be sufficient to rewire just one resistor pack?

[exxos]

Would the whole bus have to be converted to 3.3V to test this, or would it be sufficient to rewire just one resistor pack?

One could be tried first I guess.. but all the data & address bus would need to be tried afterwards.

EDIT:
A scope on the pins to make sure nothing is pushing back up to 5V would need to be checked as well.. The bus isolator chips may not play nice for example.