Throwing 64K ROMs into a Mac Plus ?

[tjjq44]

Hi,

I'm actually fiddeling with a spare Mac Plus logicboard I have. Sadly I don't have the original ROMs for this board. I also own a rominator board so I decided to use it. After burning the appropriate rom files (Hi & Lo) it works well.

I then thought of an improvement : I always wanted to have one machine that could behave either as a "Plus" or a "512k" depending on my needs. The chips that come with Rominator are quite large (512k each) so I decided to burn "Plus" and "512k" ROMs onto the chips and use a switch to toggle from one to the other (A19 is connected to VCC or GND).

It works flawlessly in "Plus" mode but if I switch to "512k" mode, when I turn it on I hear an unpleasant shrill sound and I see large vertical stripes on the screen.

Maybe I made a mistake configuring the whole thing but one question remains unanswered before I pull my hair on this : has anyone successfully booted a Mac Plus using stock 64k ROMS (from a mac 128k or 512k) ? Is it just impossible to make it work?

EDIT: In desperation, I've just tried to put a resistor in the "One row" location in order to eliminate a incompatibility between 64K ROM and 1024KB memory, and... it works!!! I can now choose if I use a 512k or 512ke on the same logicboard.

Remaining question : What could prevent 64K ROM to boot the Mac when more than 512K of RAM is attached?

 

[demik]

The 64k ROM assumes that the max RAM possible is 512. Since video and sound buffers are at the top of memory, this can't work

The 68k ROM make the CPU writes at the top of 512kB of RAM, and your logic board video/sound circuit reads at the top of 1024kB of RAM

 

[tjjq44]

Ok so this implies that every third party memory expansion available for the Mac 512k at the time needed a Mac Plus ROM upgrade, I'm right?

 

[Iesca]

That's probably a safe assumption. And if you had a 512k at the time, you would probably have been to happy to have it, not just for the extra RAM accessibility, but also access to HFS without having to go through boot disk shenanigans like with the HD20. Even if you didn't have the 800k disk drive upgrade, that would have still been massively helpful (even if it did suddenly prevent you from using very early system versions).

 

[Realitystorm]

Ok so this implies that every third party memory expansion available for the Mac 512k at the time needed a Mac Plus ROM upgrade, I'm right?

The newlife adapters needed the 128k rom upgrade in order to work

 

[demik]

Ok so this implies that every third party memory expansion available for the Mac 512k at the time needed a Mac Plus ROM upgrade, I'm right?

Yes, as most of the SCSI upgrades. Some board asked for Plus ROMs, others had patched 64k ROMs in other way or another.
The Beck-Tech MacMEGABYTES used a second set of ROMs to patch the 64k ROMs

 

[tjjq44]

That makes sense, thank you everybody!

I Thought It was possible cause I read this PDF file, at page 28 it states that it is possible to upgrade a Mac 512k to 1 meg with nothing else than DRAM chips, a 74LS138 and some passive components... The guy doesn't seem to have tested it by himself apparently!