VGA feature connector problems
VGA feature connector was originally designed to be an interface between VGA card and external RAMDAC chip. This interface has been then used for many application including vidoe overlay cards and some VGA to TV converters.
VGA feature connector was quite useful way to interface to the VGa card but incread resolutions and number of colors have made it quite useless in modern VGA cards. The VGA feature connector has following limitations:
- 8 bit bus does not give enough bandwidth for mugh higher than normal VGA resolutions
- The interface is limited to 256 colors
- The interface just transfers the pixel values and the card which reads them need also other information form other sources to show the picture correctly
Problems with VGA to TV converters and overlay cards which connect to VGA feature connector
Typical problem is that the output of the card connected to VGA feature connector looks somehow scrambled, typically wrong colors in standard VGA mode. And higher resolutions the systems stops working completely. The problem in this setup is that those old hardware which uses VGA feature connector is so outdated technology that it can't properly work with modern vidoe card which are connected to faster buses than ISA. This is one disadvantage of rapid PC video card technology devellopment. If you still want to use those old hardware then you have to install an old ISA bus VGA card to your computer and live with it's limitations.
Those cards which take picture from the VGA card through VGA feature connector have a problem that feature connector carries only the pixel data. The palette information must go to that anothe card in some other way, typically the card listens to the I/O writes in ISA BUS to the VGA card's palette chip registers. If the video card is connected to VLB or PCI bus the paletter writes never go to ISA bus and that card on ISA bus wil never gat that information. Bcause the card in ISA bus can't get palette information it can't display colors right. I had this exact problem with one VGA->TV card which plugged to VGA feature connector and that product worked correctly only with VGA card which were connected to ISA bus (the colors were wrong with other VGA cards).
Anpther problem is limited number of colors and bandwidth available in VESA feature connector. It was never ment to handle more than 256 colors and more than standard VGA resolutions (though it works in a little bit higher resolutions sometimes). Matrox Millenium and many other modern video cards have VGA feature connector only activate it in standard VGA modes. That's because VGA feature connector can't properly handle higher resolutions and/or more colors. Matrox Millenium runs the standard VGA resolutions through CRT controller and 8 bit display bus like normal VGA card and sends data to the feature connector if enabled. In all all higher resolutions Matrox Millenium uses other methods to get enought bandwidth (the RAMDAC reads memory directly through wider bus).
Tomi Engdahl <[email protected]>