IBM Color Graphics Adapter: Difference between revisions

IBM Color Graphics Adapter
	File:IBM CGA placeholder.png
	The IBM Color Graphics Adapter (CGA), 8-bit ISA card with TTL RGBI and composite NTSC outputs
Manufacturer	IBM
Type	ISA video display adapter
Release date	August 1981
Discontinued	~1987
Price	US$300 (1981)
Interface	8-bit ISA (PC bus)
Compatible	IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170), PC-compatible clones
Model	IBM CGA / 1501486
Connectivity	DB-9 TTL RGBI (drives the IBM 5153); composite NTSC RCA; light-pen header
Part number	1501486

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Revision as of 10:07, 21 May 2026

The IBM Color Graphics Adapter (CGA) is the original 8-bit ISA colour graphics card IBM released alongside the IBM PC (5150) in 1981. It uses a Motorola 6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the IBM 5153) or a composite TV through an NTSC encoder.

Video Modes

**CGA video modes**
Mode	Resolution	Colours	Notes
0	40 × 25 text	16 fg / 8 bg	Composite or RGBI
1	40 × 25 text	16 fg / 8 bg	Colour burst suppressed
2	80 × 25 text	16 fg / 8 bg	8 × 8 character cell
3	80 × 25 text	16 fg / 8 bg	Colour burst suppressed
4	320 × 200 graphics	4 from two fixed palettes	Palette 0: cyan/magenta/white; Palette 1: green/red/yellow
5	320 × 200 graphics	4 (greyscale on composite)	Composite "alternate" palette
6	640 × 200 graphics	2 (background + foreground)	Highest resolution

Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block "mode 7", composite NTSC artefact colour, "8088 MPH"-style techniques), but they are not part of the BIOS interface.

Hardware

6845 CRTC — clock and sync generation, addressing.
16 KB video RAM (8 × 4116 DRAM on early boards, later consolidated).
TTL RGBI output on DB-9 (drives the IBM 5153).
Composite NTSC output on RCA jack (drives any NTSC TV).
Light-pen connector (rarely used).

Common Faults

Snow — the famous CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Disable by waiting for the horizontal retrace (most CGA-aware games already do this).
No video, beep code 1 long + 2 short — the CGA card is dead or unseated. Reseat first.
Missing one colour channel — bad output buffer at the DB-9, or cracked solder joint on the connector.
Smoke from a discoloured tantalum — replace the failed tantalum (typically a 22 µF / 16 V) and any series resistor that may have died with it. See ISA card capacitor positions.

Related Pages

IBM PC (5150)
IBM 5153 — the matching colour monitor
IBM Monochrome Display Adapter — the monochrome alternative
IBM PC (5150) Capacitor Replacement Guide

@@ Line 1: / Line 1: @@
+{{Infobox computer peripheral
+| name          = IBM Color Graphics Adapter
+| image         = [[File:IBM CGA placeholder.png|260px]]
+| caption       = The IBM Color Graphics Adapter (CGA), 8-bit ISA card with TTL RGBI and composite NTSC outputs
+| manufacturer  = IBM
+| type          = ISA video display adapter
+| release_date  = August 1981
+| discontinued  = ~1987
+| price         = US$300 (1981)
+| interface     = 8-bit ISA (PC bus)
+| compatible    = [[IBM PC (5150)]], [[IBM PC XT (5160)]], [[IBM PC AT (5170)]], PC-compatible clones
+| connectivity  = DB-9 TTL RGBI (drives the [[IBM 5153]]); composite NTSC RCA; light-pen header
+| model         = IBM CGA / 1501486
+| part_number   = 1501486
+}}
 The '''IBM Color Graphics Adapter''' ('''CGA''') is the original 8-bit ISA colour graphics card IBM released alongside the [[IBM PC (5150)]] in 1981. It uses a [[Motorola 6845]] CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the [[IBM 5153]]) or a composite TV through an NTSC encoder.