Tonight I wired up a 74HCT157D according to the schematic for the
garlic switcher, hoping to use that to be able to switch between 68000 and 68030....
IMG_20200730_204814.jpg
It looks like a bit of a science project, but the wiring passes check and double check. I cut the traces on the relocator for E and BGACK for the right hand (TF) socket, but not for the left, because I still want those for the TOS206/IDE board. So those pins are bent out with wires soldered on on the 68000 on top, although it doesn't really show very well in the photo.
Unfortunately, it doesn't work. With the "switch" (just a wire to GND in the breadboard setup) in either position, I get a very dark (a very analogue looking dark) screen with some garbage when I boot in mono (heh, with colour timings once again) and a white screen with green and pink garbage if I boot in low res.
I verified that the multiplexer is doing what it's supposed to do, by pulling out the 68000 and verifying it'll boot off the TF (and that it acts like no CPU is present when "switched" the other way), and also that it'll boot the 68000 if I pull out the TF.
I was initially surprised that the garlic design switches only E and BGACK, where the old
PAK switcher linked by
@czietz was switching E, BG, and BR, but I figured I'd try it out anyway. I probably could have saved myself some time (okay not really, because I would have built this anyway) by testing this on the H4, because sure enough, if I put in two CPUs, and bend out E and BGACK pins on one of them, it won't boot. Bend out the BR pin in addition and it boots fine.
So it would seem that the Amiga is tolerant of two CPUs with BR connected, but the ST is not. This is a quad-input multiplexer, so I think it can do the job, although I'll need to change from switching BGACK to switching BG, due to directionality constraints. But that is a job for tomorrow. :coffee: