Hello,

In the meantime I managed to open the display case and found out is a CGA LCD... If anyone is interested, is a SHARP LM64135Z / F0047 HCN-24V

So I'm trying to revive this wonderful piece of technology by putting a LCD driver with a modern LCD. But, before doing this, I need to find a way to convert the CGA output to VGA signal.
I tested with the simplest circuit I could find, that uses only some resistors and a 15KHz monitor and it worked. The problem is that something is happening when is booting in MS-DOS (bootdisk). The lines seem to overlap each other, so I see only half the text.

When I start-up the computer all BIOS and POST text is showing properly.
Even in the first seconds of MS-DOS booting (from FDD), the "Starting MS-DOS" font is showing good, then it gets cut.

I tried the MODE command in MS-DOS but is no difference (except text is bigger/smaller, but still cut).

My guess is that the font is displayed full and properly, but somehow the lines' vertical distance gets crazy and overlap the above line.

Does anyone have any idea what could be the issue here?

Hi there!

I've tried to look up the specs for this laptop..
https://www.saintummers.at/nixdorf/nc8810.html

The original display seems to be very crisp.
Does it really just use CGA (640x200 mono)? Or 640x400 (mono) or higher?

CGA compatible graphics with higher resolution existed.
Especially some laptops, like the Toshiba T3100 had it.

Olivetti/Logabax/AT&T/Toshiba 640x400 hi-res graphics mode
https://en.wikipedia.org/wiki/Toshiba_T3100

In BIOS I can choose "mono", "color 40" and "color 80" - which these numbers are in fact the resolution of the text columns. I don't know the resolutions that supports, but my external monitor is switching to something like 760x480 (don't remember exact numbers, but similar)
The photos above are taken from a modern external monitor.
Interesting thing is if I choose MONO from BIOS, is even worse... All screen is black, except the first line (that says Starting MS-DOS in my case). First I thought the computer hanged but in fact some black layer covered all the text below the first line.
Could this be because of the improvised converter?
I used the circuit from this video:
https://www.youtube.com/watch?v=1KH1D_qqID0

OK I think I found the problem, it seems that MS-DOS 5.0 and above are too new for this computer.... at least using the external CGA conversion, because on the other forum, those guys seem to run a newer dos version with the internal LCD.
I tried with MS-DOS 3.3 and MS-DOS 4.01 boot disks and these worked properly

Hi again! 😀

It's worth a try, imho. My favorite retro DOS was PC-DOS 3.30, btw.
Just loved that parrot and the cool manuals! 😉

Btw, you can also run a copy of MS-DOS 6.x and press F5 during boot.
That will give you a clean boot without any drivers.

Or you can try to REM out these lines in autoexec.bat and config.sys containing drivers:

Ansi.sys, ega.sys, MODE CON, display.sys etc.

These drivers do change the command prompt/fonts/code page etc.
Without these, the DOS prompt will (should) use the same screen mode as the BIOS during POST.

That being said, you may already know these.
It's just that I don't know what you already know. 😅

Good luck!