DB-9

DB-9 (technically DE-9) is a 9-pin D-subminiature connector used for serial communications, digital video output, and game controller connections on personal computers and peripherals. The designation "DB-9" is a widespread misnomer — the correct name is DE-9, where "D" indicates the D-sub connector family and "E" denotes the shell size, but "DB-9" became so entrenched in common usage that it remains the more recognized term.

In the context of CRT monitors, the DE-9 connector is significant as the standard video output for IBM CGA, MDA, and EGA graphics adapters, carrying TTL-level digital RGB signals to compatible CRT displays.

History

The D-subminiature connector family was introduced by Cannon (now part of ITT Inc.) in 1952. The connectors were named for their characteristic D-shaped metal shield, and at the time of their introduction they were among the smallest connectors used on computer systems.

The DE-9 variant gained widespread use in the 1980s when IBM adopted it for serial communications on the IBM PC/AT, replacing the larger DB-25 connector that had been standard on the original IBM PC. As the RS-232 standard evolved and many of its defined signals became obsolete, the smaller 9-pin connector proved sufficient for the two signal wires and ground needed for most serial communication.

Simultaneously, IBM used the DE-9 connector as the video output on its CGA (1981), MDA, and EGA (1984) graphics adapters, carrying TTL-level digital RGB signals to direct-drive CRT monitors like the IBM 5153 Color Display.

Design

The DE-9 connector uses an E-size D-shaped metal shell containing 9 pins (male) or 9 sockets (female) arranged in two rows — five pins in the upper row and four in the lower row. The D-shape ensures correct orientation and prevents reversed insertion. Panel-mounted connectors are typically secured with 4-40 UNC threaded inserts (known as "jackscrews") that mate with thumbscrews on the cable connector for a secure connection.

Use as a Video Connector

CGA (Color Graphics Adapter)

The IBM CGA card (1981) used a female DE-9 connector to output 4-bit TTL-level RGBI (Red, Green, Blue, Intensity) signals to a direct-drive CRT monitor. The four color bits were output directly to the connector, allowing up to 16 colors. Compatible monitors include the IBM 5153 and various third-party RGBI displays.

CGA/EGA DE-9 Pinout

Pin	CGA Signal	EGA Signal
1	Ground	Ground
2	Ground	Secondary Red
3	Red	Primary Red
4	Green	Primary Green
5	Blue	Primary Blue
6	Intensity	Secondary Green
7	Reserved	Secondary Blue
8	Horizontal Sync	Horizontal Sync
9	Vertical Sync	Vertical Sync

EGA (Enhanced Graphics Adapter)

The IBM EGA (1984) used the same DE-9 connector and maintained backward compatibility with CGA monitors. When operating in EGA's native modes, pins 2, 6, and 7 were repurposed to carry secondary RGB signals, expanding the color palette from 16 to 64 colors. EGA monitors could detect the mode and interpret the additional signals accordingly.

MDA (Monochrome Display Adapter)

The IBM MDA also used the DE-9 connector but carried only a monochrome TTL signal (green or amber, depending on the monitor) plus horizontal and vertical sync. The IBM 5151 monochrome monitor was the standard display for MDA.

Tandy and Other Systems

The Tandy 1000 and IBM PCjr used the same DE-9 connector with CGA-compatible RGBI pinout for their built-in graphics adapters, allowing connection to standard CGA monitors.

Transition to VGA

When IBM introduced VGA with the PS/2 in 1987, the DE-9 was replaced by the DE-15 (VGA) connector for analog RGB video. Some early third-party VGA cards and monitors briefly used DE-9 connectors with an analog VGA signal, but these were quickly supplanted by the DE-15 standard.

Use as a Serial Port

The DE-9 connector became the standard for RS-232 serial communication on IBM PC/AT-compatible computers, replacing the DB-25. RS-232 serial ports were used extensively for connecting modems, mice, printers, and other peripherals. While now largely supplanted by USB, RS-232 serial communication via DE-9 remains in use for industrial equipment, networking hardware, and scientific instruments.

Use as a Game Port

Atari popularized the DE-9 connector for joystick and game controller connections on the Atari 2600 (1977) and Atari ST computers. The same connector and basic pinout was adopted by Commodore (C64, Amiga), Sega (Master System, Genesis/Mega Drive), and other home computer and console manufacturers. This "Atari-style" joystick port remained a de facto standard for game controllers through the early 1990s.

See Also

• DE-15 (VGA) — 15-pin VGA connector that replaced DE-9 for video
• EIAJ-8 — 8-pin rectangular connector for TTL RGB on Japanese equipment
• TTL RGB — Digital RGB signal format carried over DE-9 on CGA/EGA
• BNC — Professional video connector