On Tuesday (01/18/2022 at 02:01PM -0600), Mike Katz wrote: > I think it might be easier to modify the 680 prom for the I/O addresses of > the board rather than modify the board to match the ROM.
Agreed-- except the goal, which I failed to elaborate on, is to come up with an Altair 680 development environment so that someone can port some code to the platform without having the real thing. I wanted to make that environment as close to real as possible (without having front panel switches and LEDs)-- which means having the I/O in the same place as the original as well as the authentic PROM code running. > Especially if the address decoding for the I/O is done in PAL (10L8 for > example). No PALs on the board but there is a bipolar PROM (82S129). I'm not adverse to making a new one of those or bodging something that drops into that socket to modify the decoding if neccessary. I was just hoping to not have to butcher the board itself too much. > Some 6800 address decoding was done with 74LS138s. This had the potential > to be inefficient in terms of memory usage or if the '138s were cascaded > then propagation delay could become an issue. Yes. This seems to be a limited function CPU board and I suspect it takes that approach just to get the four PROMs and I/O decoded very coarsely. Chris -- Chris Elmquist
