Hi all,
I have a firends ROM3 IIgs here which refrains from booting properly. It goes straigt into a flickering checkerboard like so: https://youtu.be/bRoEhC-HIzo
Booting into self-test gives a more colored checkerboard with some random movements: https://youtu.be/Xb7L7TnC-GA
Initially I thought that might be a ROM issue, but after swapping the ROMs for known good ones as well as the CPU, it's clear that there's more to it.Do you, the nice people of Applefritter, have an idea what/where to look for? No chip is getting hot or even remarkible warm.
My uneducated guess is, there's an issue already with the RABUS adressing - crossing my fingers that it's not the MEGA2... are the HCT245 buffers known to fail?
Thanks in advance for all your insights and tips & cheers,Axel
Ok, I'm replying to myself, just in case somebody else comes about the same issue:
Check the data-bus at the buffers as well as on the RAM. Then all RAM control lines. In my case it was /RAS which looked terrible in the scope... so it was a dead resistor (100 ohm, R53 IIRC) which needed to be replaced. After that the board happily self-tested and all is good again.
Cheers,Axel
Alex_M
Thanks for the post
I too am experiencing a checkerboard pattern. After scoping out the address lines it seems that A8 through A15 are all low. The HCT245 seems to be the impass to the Mega chip. Hopefully when the part arrives and is installed the problem will go away.
I don't know about in the IIgs specifically, but 74___245 chips are often heavily loaded and/or the "ftont line". So that would be something I'd check first before pointing at the MEGA2. The other thing is 245s are available, the MEGA2 is unobtanium. If it is bad, you're basically going to need another motherboard. Unlike the //e and //c where there is work in progress to eventually be able to create replacements for the currently unobtanium IOU and MMU chips, the MEGA2 as far as I know nobody has thought to tackle yet. I suppose it might be possible to do with a big enough FPGA, but it would be a herculean project.
HCT245 replaced and same issue is present. Thinking it might be the FPI not allowing the address line to talk to the MEGA. I can mash the reset a bunch of times and get some pulses out of the CPU. Nothing changes on the monitor though. FPI controls the ABUS and DBUS. DBus enable has a nice clock enable signal but ABUS not so much. Anways thats where I am at. Not sure what to try next. I guess I need another iigs to see what a good signal looks like.