Enhanced Graphics Adapter - Specifications

Specifications

The EGA uses a female 9-pin D-subminiature (DE-9) connector which looks identical to the CGA connector. The hardware signal interface, including the pin configuration, is largely compatible with CGA. The differences are in the repurposing of three pins for the EGA's secondary RGB signals: the CGA Intensity pin (pin 6) has been changed to Secondary Green (Intensity); the second ground of CGA (pin 2) has been changed to Secondary Red (Intensity), and pin 7 (Reserved on the CGA) is used for Secondary Blue (Intensity). If the EGA is operated in the modes having the same scan rates as CGA, a connected CGA monitor should operate correctly, though if the monitor connects pin 2 to ground, the shorting of the EGA's Secondary Red (Intensity) output to ground could conceivably damage the EGA adapter. Similarly, if the CGA monitor is wired with pin 2 as its sole ground (which is poor design), it will not work with the EGA, though it will work with a CGA. Finally, because of the use of the CGA's Intensity pin as Secondary Green, on a CGA monitor connected to an EGA, all CGA colors will display correctly, but all other EGA colors will incorrectly display as the standard CGA color which has the same values for the g, R, G, and B bits (ignoring the r and b bits.) Conversely, an EGA monitor should work with a CGA adapter, but the Secondary Red signal will be grounded (always 0) and the Secondary Blue will be floating (unconnected), causing all high-intensity CGA colors except brown to display incorrectly and all colors to perhaps (but probably not) have a blue tint due to the indeterminate state of the unconnected Secondary Blue.

The IBM 5154 EGA monitor has a special IBM 5153 CGA compatibility mode when operating with CGA sync signals, and it will automatically change to the CGA pinout to avoid all of the mentioned problems when operating in this mode.