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In 1984, IBM introduced the Enhanced Graphics Adapter (EGA) for the IBM Personal Computer, which was designed to recover some of the market share lost to Hercules and other manufacturers of video cards. These non-IBM products had higher screen resolutions or supported more colors for graphics than what IBM offered on the original Color Graphics Adapter (CGA).
The EGA adapter allowed resolutions up to 640x350, and up to 64 fixed colors using a TTL monitor interface, which allowed four different intensity levels from each video gun (Red, Green and Blue). Although not all of that color depth was actually available at all video resolution modes that IBM provided, EGA was generally an improvement over the CGA system, as CGA supported only two-color (white/black) graphics at 640x200 and four-color graphics (white, black, magenta and cyan) in 320x200 mode. A main complaint about the CGA system was that the color choices for the graphics modes were not adjustable.
EGA was never an enormous success, partly because it required a special EGA-capable monitor and some of the EGA video modes were difficult to write software for. Other video card makers were providing full 16-color graphics as a refinement to both of the CGA graphic resolutions, which meant that an existing CGA monitor could be re-used with a newer video card and better graphics quality could be obtained. Most consumers seemed satisified with the CGA 640x200 or 320x200 graphics resolutions until the late 1980s when VGA became widely available and more affordable. Some computer makers with embedded video hardware continued to deliver enhanced CGA video systems as late as 1990, years after all EGA production had ceased.
As in the CGA system, EGA kept horizontal and vertical sync information separate from the video signals. Dual mode monitors were produced that could accept CGA or EGA signals, telling the two systems apart by the fact that EGA used an inverted (negative polarity) vertical sync signal.
EGA/CGA monitors and adapters are not compatible with VGA hardware, introduced in 1986. VGA systems have no separate intensity signal like CGA, nor does it have multiple digital brightness signals like EGA. In VGA, each of the gun color signals (Red, Green and Blue), is variable, or Analog. The stronger the signal sent by the video controller for a given color, the brighter that color is displayed, allowing for a wide range of colors to be allowed by the monitor, even if the early VGA cards did not have sufficient RAM or other components to allow large numbers of colors to be displayed. That was a limitation of the video adapter and not the monitor itself.
EGA and the older CGA system use a subminature D 9-pin connector, commonly known as a DB-9. The computer typically has a female socket.
Because of the analog nature of VGA signals, additional shield ground connections were required in the cabling system, so a subminature D 15-pin (DB-15) connector was used for VGA.
| Pin | Name | Notation | Description | Level | Direction |
|---|---|---|---|---|---|
| Ground | Bi-directional | ||||
| Red Bit 0 | Red Component of a RBG-encoded video signal | TTL* (Combination of R0 and R1 dictate four brightness levels of the Red gun) | To Monitor | ||
| Red Bit 1 | Red Component of a RBG-encoded video signal | TTL* (Combination of R0 and R1 dictate four brightness levels of the Red gun) | To Monitor | ||
| Green Bit 1 | Green Component of a RBG-encoded video signal | TTL* (Combination of G0 and G1 dictate four brightness levels of the green gun) | To Monitor | ||
| Blue Bit 1 | Blue Component of a RBG-encoded video signal | TTL* (Combination of B0 and B1 dictate four brightness levels of the Blue gun) | To Monitor | ||
| Green Bit 0 | Green Component of a RBG-encoded video signal | TTL* (Combination of G0 and G1 dictate four brightness levels of the Green gun) | To Monitor | ||
| Blue Bit 0 | Blue Component of a RBG-encoded video signal | TTL* (Combination of B0 and B1 dictate four brightness levels of the Blue gun) | To Monitor | ||
| Horizontal Sync | Horizontal Sync clock | TTL* Positive Polarity | To Monitor | ||
| Vertical Sync | Vertical Sync clock | TTL* Negative Polarity | To Monitor |
*TTL signals operate in the range 0.0VDC to 5.0VDC. Levels from 0.0VDC to 0.8VDC are considered to be an "OFF" or "0" state. Levels from 2.0VDC and 5.0VDC are considered to be an "ON" or "1" state. Levels between 0.8VDC and 2.0VDC are not defined and their use is not predictable. (Some vendors state that the TTL "ON" state begins at 2.4VDC.)
In cables with separate BNC connectors, the colors shown in the table represent the wire colors most commonly used for the various signals, except for Ground, which is built into each cable and connector in order to provide shielding.
| Red Color | R1 | R0 | Intensity Sample |
|---|---|---|---|
| No Red | OFF | OFF | |
| Dim Red | OFF | ON | |
| Medium Red | ON | OFF | |
| Bright Red | ON | ON | |
| Green Color | G1 | G0 | Intensity Sample |
| No Green | OFF | OFF | |
| Dim Green | OFF | ON | |
| Medium Green | ON | OFF | |
| Bright Green | ON | ON | |
| Blue Color | B1 | B0 | Intensity Sample |
| No Blue | OFF | OFF | |
| Dim Blue | OFF | ON | |
| Medium Blue | ON | OFF | |
| Bright Blue | ON | ON |
| Mode | Dot Clocking Frequency | Dot Data Polarity | Horizontal Clocking Frequency | Horizontal Polarity | Vertical Clocking/Refresh Frequency | Vertical Polarity | Active Dots | Active Lines |
|---|---|---|---|---|---|---|---|---|
| CGA | Positive | 15.75kHz | Positive | 50Hz | Positive | 640 | 200 | |
| EGA | Positive | 21.8kHz | Positive | 60Hz | Negative | 640 | 350 |
[Copyright 2001,2002,2003 Frank Durda IV, All Rights Reserved. Mirroring of any material on this site in any form is expressly prohibited. The official web site for this material is: http://nemesis.lonestar.org Contact this address for use clearances: clearance at nemesis.lonestar.org Comments and queries to this address: web_reference at nemesis.lonestar.org]
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