could you elaborate on that, what parameters are used in cgxmod for, say, 960x540 ?
I've tried many combinations, but monitor detects input as 640x480 or 720x576 and similar.
I never heard of this mode. Does your monitor even support it? You can only use modes which your monitor supports. Find the list of supported modes and the corresponding horizontal and vertical frequencies in the manual or on the manufacturer's web page.
In CGXmode select a mode from the list which has a similar height, then overtype the resolution and change sync sizes and pixel clock until the frequencies match the desired ones as close as possible.
Here is a cgx monitor file with a 1280x768 mode.
