I have a model 1 japanese mega cd. It boots fine, but when I enter the options screen I get a completely black screen, and pressing start I get some blue rectangles.
Similarly, when I boot a CD I get completely garbled graphics.
I assumed I had some of the ram (either program ram or one of the two word ram) broken or with traces disconnected.
To identify which, I wrote this test program
https://github.com/fabiodl/megacdTest/
which I start using an everdrive. The program sets fills the memories with a known pattern,and then reads them back.
To my surprise, all the memories pass the test, both from the main cpu and from the sub cpu.
The mega cd 68K seems fine as well. As by program, it is able to communicate back to the genesis 68K, to blink the two LEDs and to receive interrupts (int2 from the main cpu and int 3 from the gate array)
I also tried to start the bios from the everdrive to see if there were problems with the ROM but the behavior is the same. Using the US bios I get no spinning SEGA logo at all.
What could be possibly wrong?
Similarly, when I boot a CD I get completely garbled graphics.
I assumed I had some of the ram (either program ram or one of the two word ram) broken or with traces disconnected.
To identify which, I wrote this test program
https://github.com/fabiodl/megacdTest/
which I start using an everdrive. The program sets fills the memories with a known pattern,and then reads them back.
To my surprise, all the memories pass the test, both from the main cpu and from the sub cpu.
The mega cd 68K seems fine as well. As by program, it is able to communicate back to the genesis 68K, to blink the two LEDs and to receive interrupts (int2 from the main cpu and int 3 from the gate array)
I also tried to start the bios from the everdrive to see if there were problems with the ROM but the behavior is the same. Using the US bios I get no spinning SEGA logo at all.
What could be possibly wrong?