Hi everyone, good night!
First al all, a big hug to all, I have followed the group but I couldn’t participate too much due to time problems… a lot of work (luckily) and a big lockdown here have limited me in several aspects but I’m organizing myself to try to return to the things that I love. Between them I have my retro projects 
I have returned to the repair project of an old Monroe OC8820. This isn’t the first post about it.
- Trying to find a Monroe computers list
- Micropolis 1015F drive connected to PC
- Monroe OC-8820 repair project (part 1) - #2 by NoLand
At this point, I have achieved the following points:
- I have been able to use the Micropolis drive with a common PC, and prepare a floppy with the CP/M version that (I think) should work with the computer.
- I dumped the BIOS, but sadly I couldn’t find an emulator to test it.
After the last point, I have thought that would be great to have a memory map of the computer, but I found that it could be more difficult than I imagined.
- I have detected other chips that could provoke the error. Two 74N258 and two 74S257.
This is the current state of the board.
I have tried to find other machines of the same period based on Z80 with a memory mapper based on two adders (74LS283), but I couldn’t find anything. I will remove all memory chips (4116) in order to check why the multiplexers have failed, and if it’s possible try to find a more modern replacement.
My main goal with this topic was to share my findings and let you know that I don’t have forgotten the group.
I will try to keep the topic updated, any question, comment or additional information is welcome.