Well, I got my cartridge. It actually does something, which is more than I expected.
Upon power up, it says "I6 Bus Error handling failed"
Next, it moves to a screen with the heading "Testing MFP, Glue Timing, Video" ...and then prints...on the next line..
"T0 MFP Timer"
It makes it no further than that. The screen simply stays locked on that image. No key press will move past the screen. I would also note that there are still vertical bars in the image.
Per the service manual (thanks!) the diagnostic cart also writes to RS232 the same information (or is it more information?) it prints on screen and looks to it for input there as well. Seems that might allow me to push space or another input after the error message is received to move past it...if the keyboard is caught up in the error in my ST. It may be as the referenced MFP does seem to handle keyboard interrupt.
That being said, I'll have to either dig up, build or buy an atari to 9 pin serial adapter to hook up to the RS232.
Any other suggestions? Or is protocol here to just remove/replace the 68901 MFP? (on that note, it seems that I can find tons of 68901P, but I can't find anywhere that tells me what that P is for/if it will make a difference).
***Also, Arne, thank you, and sorry. I had already ordered from Best by the time a saw your post.