EGA Switch Settings

 This topic discusses the switches on older IBM-logoed EGA cards ONLY.
 On non-IBM EGA cards, there may be more or fewer switches and they may
 have different meanings.

 The DIP switches on the rear of the EGA indicate which display monitor
 is connected to the card and which (if any) other adapters are in the
 system at the same time.  The switches are mirrored in memory.  See
 EGA Data Areas.

 There is one CRITICAL thing to watch for:  never plug a Monochrome monitor
 into the EGA adapter unless the switches are set to accept it.  A wrong
 setting will actually do physical damage to the monitor!

 In the following table, Primary indicates the adapter/monitor which is
 first enabled when the computer is turned on.  Secondary indicates an
 optional second adapter/monitor which may be connected.

ON ╓4┬3┬2┬1╖ Primary EGA connected to 40x25 color monitor
 ↑ ║▀│▄│▄│▀║ Secondary MDA
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary EGA connected to 80x25 color monitor
 ↑ ║▀│▄│▄│▄║ Secondary MDA
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary EGA connected to enhanced color monitor (CGA emulation)
 ↑ ║▄│▀│▀│▀║ Secondary MDA
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary EGA connected to enh. color monitor (best resolution)
 ↑ ║▄│▀│▀│▄║ Secondary MDA
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary EGA connected to Monochrome monitor
 ↑ ║▄│▀│▄│▀║ Secondary CGA connected to 40x25 color monitor
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary EGA connected to Monochrome monitor
 ↑ ║▄│▀│▄│▄║ Secondary CGA connected to 80x25 color monitor
   ╙─┴─┴─┴─╜

ON ╓4┬3┬2┬1╖ Primary MDA
 ↑ ║▀│▀│▀│▀║ Secondary EGA connected to 40x25 color monitor
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary MDA
 ↑ ║▀│▀│▀│▄║ Secondary EGA connected to 80x25 color monitor
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary MDA
 ↑ ║▀│▀│▄│▀║ Secondary EGA connected to enh. color monitor (CGA emulation)
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary MDA
 ↑ ║▀│▀│▄│▄║ Secondary EGA connected to enh. monitor (best resolution)
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary CGA connected to 40x25 color monitor
 ↑ ║▀│▄│▀│▀║ Secondary EGA connected to Monochrome monitor
   ╙─┴─┴─┴─╜
ON ╓4┬3┬2┬1╖ Primary CGA connected to 80x25 color monitor
 ↑ ║▀│▄│▀│▄║ Secondary EGA connected to Monochrome monitor
   ╙─┴─┴─┴─╜

█▌EGA Jumpers▐█
  The three jumpers on the EGA control various features of the card.  The
  jumper/connectors are used as follows:
    ╔P1═╗
  1 ║┌•┐║ Supports enhanced color monitor
  2 ║└•┘║ (640x350 and 64 color capability)
  3 ║ • ║
    ╚═══╝
    ╔P1═╗
  1 ║ • ║
  2 ║┌•┐║ Supports color monitor only
  3 ║└•┘║ (640x200 and 16 color capability)
    ╚═══╝

    P2 (not shown) is a connector for a light pen.

    ╔P3═╗
  1 ║┌•┐║ Sets EGA I/O ports to 3xxH
  2 ║└•┘║ (normal setting)
  3 ║ • ║
    ╚═══╝
    ╔P3═╗
  1 ║ • ║
  2 ║┌•┐║ Sets EGA I/O ports to 2xxH
  3 ║└•┘║ NOT supported by EGA BIOS
    ╚═══╝

See Also: Equipment List
          EGA Data Areas
          Switch Settings
          EGA Video Adaptor Pin Outs
                                    -♦-