Atari Mega ST 1 Rev 1 no boot

[Badwolf]

I just realised it's the exact same mod...

I like the design. I assume the other lead plugs into the ROM sockets to provide the datalines?

BW

[rubber_jonnie]

I just realised it's the exact same mod...

I like the design. I assume the other lead plugs into the ROM sockets to provide the datalines?

BW

It's been a while, so I don't fully recall, but it isn't plugged into the ROM sockets, but some sockets on ICs next to them:

Screenshot 2025-02-27 at 16.11.56.png

I think U35 and U32 which are 74LS244's which are buffers. The rest i.e the small board replaces the resistors.

[Dwh18]

Mine had extra connections to some points in the motherboard. Its and interesting mod since it uses the 1Mb onboard.

[Badwolf]

It's been a while, so I don't fully recall, but it isn't plugged into the ROM sockets, but some sockets on ICs next to them:
I think U35 and U32 which are 74LS244's which are buffers. The rest i.e the small board replaces the resistors.

Ah yes, of course. The ROM would be on the wrong (CPU-side) databus.

Shame it didn't work, but could that bit a hint: are those data buffers functioning properly after the removal?

BW

[rubber_jonnie]

It's been a while, so I don't fully recall, but it isn't plugged into the ROM sockets, but some sockets on ICs next to them:
I think U35 and U32 which are 74LS244's which are buffers. The rest i.e the small board replaces the resistors.

Ah yes, of course. The ROM would be on the wrong (CPU-side) databus.

Shame it didn't work, but could that bit a hint: are those data buffers functioning properly after the removal?

BW

IIRC they had pin headers or sockets on top of them, not great. I didn't need to replace them when I removed the mod, so I think mine at least were ok. Doesn't mean they all will be.

[Dwh18]

Have you left the computer booting for like a minute? It takes a pretty long time for it to show desktop if no disk or drive is attached.

Left it for 5m and nothing. I think next step is to analyze serial output, already ordered a cable

[rubber_jonnie]

Have you left the computer booting for like a minute? It takes a pretty long time for it to show desktop if no disk or drive is attached.

Left it for 5m and nothing. I think next step is to analyze serial output, already ordered a cable

Did you remove the small adapter board? Mine wouldn't work until I did so, no RAM.

[Dwh18]

Left it for 5m and nothing. I think next step is to analyze serial output, already ordered a cable

Did you remove the small adapter board? Mine wouldn't work until I did so, no RAM.

Yes, same here … no ram until all resistors went back to their original place in motherboard

[rubber_jonnie]

Did you remove the small adapter board? Mine wouldn't work until I did so, no RAM.

Yes, same here … no ram until all resistors went back to their original place in motherboard

Great.

[Dwh18]

Yes, different behavours with Hi and Lo Res .... Diag only loads in High Res monitor, and previous RAM error (white border whith black screen) only appeared in Low Res monitor, so RAM error I think it's done. the only patch I can see now on the motherboard seem's to be related with FDD I'll try to remove it and check behaviour. Tks!

Definitely looking like a RAM issue. I'd definitely recommend using a serial cable, it will tell you an awful lot about the RAM during boot.

One thing I would add here is that the Mega ST mainboards are pretty fragile, so handle with care, I had one board where I plugged an IDE adapter into the MegaBus connector and it popped 2 vias and broke it.

Quick question.... serial output only works with Diag cart on right? or a regular boot should show some output?