Display adapters, available through Apple resellers and the third-party companies, contain a sense code that tricks the Macintosh into believing that the display is fixed frequency 16-in. or 19-in. display. Without the adapter, the display will be fixed at a 640x480 resolution.
Computers and Video Cards Not Requiring Video Adapters
Power Macintosh family
Macintosh Centris
Macintosh Quadra
Macintosh LC 475
Macintosh Performa 475 and 476
Macintosh Display Card 24AC
Macintosh 630 Family
Computers and Video Cards Requiring Video Adapters
Macintosh II Family
PowerBook computers before the PowerBook 3400
Duo Dock and MiniDock*
Macintosh Performa Computers (except 475 and 476)
Macintosh LC, LC II, LC III
Video Cards - 4*8, 8*24, 8*24GC
* - The Duo Dock does not require a video adapter to change resolutions when System 7.5 is installed.
Display adapters are available from several third-pary companies such as Enhanced Cable Technology, Griffin Technology, and MacAdapt.