I wanted to use the VDC output of the C128 with 80 column mode. This article describes a simple circuit, which doesn't need a voltage supply. I tested it on a breadboard with some jumper wires:
Works pretty well:
The TFT display is a Dell U2410. It doesn't work with all monitors, e.g. my Hyundai ImageQuest L17T says "frequency too high". I guess it means to low, because the HSync is just 15 kHz, which is not a standard VGA frequency. And it is very much shifted to the right, maybe the polarization of the HSync pulse is wrong. But I could shift it to the right with the OSD of the monitor, so that all 80 columns are visible on screen.
This is how it looks in VICE:
Except for "dark yellow" (the color names are from page 35 of the C128 Programmer's Reference Guide). The reason is that this is a special case and displayed as brown by CGA monitors.
Compared to the original schematic I changed the resistors. 1k and 1.2k were to dark, I used 330 ohms for all resistors. And I didn't use the second row of diodes from the VGA output to the intensity, there were no difference with my monitor. This is the full modified schematic:
C128 color test program: color.prg
10 PRINT CHR$(147); 20 FOR I=1 TO 16 30 READ C$ 40 COLOR 5,I 50 PRINT CHR$(18)+" C O L O R "+CHR$(146)+" COLOR "; 60 COLOR 5,4 70 PRINT C$ 80 NEXT 1000 DATA BLACK,WHITE,DARK RED,LIGHT CYAN 1010 DATA LIGHT PURPLE,DARK GREEN,DARK BLUE,LIGHT YELLOW 1020 DATA DARK PURPLE,DARK YELLOW,LIGHT RED,DARK CYAN 1030 DATA MEDIUM GRAY,LIGHT GREEN,LIGHT BLUE,LIGHT GRAY
2. Februar 2013, Frank Buß