A IIGS owner had asked for help on FB and I offered to help thinking it may be something simple they just weren't able to do themselves. Turns out my thinking was flawed!! This board has a problem and I'm running out of ideas and could use some help. The system came with a REMicro ROM01 chip and this is a ROM01 board.
The intial problem was the computer boots (with nohting connected) to the check startup device screen. This is where things start to go south... the alterntate chararacter set characters do not appear correctly. For example this is how the screen looks:
So that seems clear the alternate characters map isn't getting used correctly.
Running the system test restuls in an error of:
System Bad: 03011500
Appears to be a problem with bit0 of the soft switch at $C015. But this is where things start to get confusing...at least for me.
If I break the startup so I could get to a prompt there's a bunch of crap being "typed" in.
If I attache a 5.25 drive to the drive port and boot computer inspector boots, gets to the main menu then starts responding to key presses. But there's no ADB nor //e keyboard connected. Because there's no connected keyboard but keyboard data I suspect the ADB micro or more likley ADB GLU could be pumping data onto the data bus. I though with the regular IIs a strobe signal gates keyboard data from getting onto the data bus, but... I really never studied this part of either a II or GS circuit.
Checking the ADB Micro ADB data line (at the end of the cable all the traffic is a repeating message that looks like this:
This is my best guess: looks like it's fairly clean with the computer looking for a keyboard, and after timing out waiting on a response tries again. I didn't see anything else. I also didn't check the pin directly on the ADB micro, that may be something I should check. I checked at the port which I think it on the other side of a transistor and resistor. BTW tons of TPs listed on the schematics, but finding them on the board.... not an easy task!
Other suspects are the 3 245s, but a quick check the data one appears to be functioning well, A and B signals match.
The FPI or MEGA II could also be factoring in, but I don't know how to check those. They (and the ADB chips) are just black boxes... I would love to see what's happening inside (code or logic). The SRAM checked out good, I added sockets ot make addition testing on RAM easier. But I ran through all 10 41464s on a TopMax memory check and swapped with known good chips from a PC graphics card.
I've been digging through Jim Sather's books on the II and IIe and the GS technical reference (not really a great resource in my opinion). I'm definately in new waters but have some good GSes I can check signal on to see if there's differences.
I'm also using a known good 12-851 supply, but I've also used a good AA-11j041B, I've replaced all the caps on the motherboard. I've swapped out the 816 and ROM wiht known good ones. and the board still fails.
Any thoughs on how to proceed? I don't like the soft switch error, becuase that's address decoding is internal to the Mega II, right?