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Possibly the wrong ROM page is being mapped into $FFxx. The screen would clear, and it would then try to call the BASIC 4 ROM (which isn't there) to output C/W/M ? or D/C/W/M ?. Try putting back the ROM that worked with the serial setup. As I said, you should at least see the "H" of H/D/M? that way. If you disabled the handshaking lines on the 6850, you should see the whole thing.

Hi again, Yes I tried putting the serial ROM back in. Someone also suggested I could then try writing to screen memory over serial, but once the 540 card is plugged in serial stops working!!!
Also still nothing (H or H/D/M) shows on the CRT with the serial rom installed.


N.B. Maybe I am using the wrong rom. I'm using just the standard SYN600 Rom file downloaded from Mark's site:
https://osi.marks-lab.com/software/roms.html

It lives!!!!!
I thought that maybe the problem with not getting serial working with the 540 board plugged in, and the 540 board not working in general were maybe related. I decided to pull chips on the 540 until serial started to work. then I put back chips and identified what chips, when installed, stopped the serial from working. Basically the 8T26's at 2G, 2H, 3H, 3I (surprisingly not the 8T26 at 1H though), and the 7404 at 4K...all chips tested ok though. I mentioned this to a friend and he suggested I also look at the other 7404 at 6L, and there is was...dead.

The image is very messy due to the noisy switcher PSU. I'll have to get another better one.
Thanks for all your help!

Next steps, build a keyboard and a D-13 data splitter.