From Sega Retro
DE-9 (sometimes incorrectly called DB-9) is a D-shaped connector/port with two rows totaling 9 pins in an E-size connector. The top row has 5 pins, and the bottom row has 4 pins. Due to the fact that it is a serial port, transmission is done one bit at a time. Two types of DE-9 connectors exist. A male DE-9 connector has pins, while a female DE-9 connector has holes. You can only plug a male connector into a female plug or vice versa, unless you have a DE-9 gender changer.
DE-9 ports are commonly seen on PCs as RS-232C serial ports, but they are also used on game consoles such as the Sega Mega Drive/Genesis and Master System for connecting input devices, such as gamepads. During the 1980s and early 1990s, it was an unofficial standard for gamepads and joysticks, meaning controllers were interchangeable between the various computers and consoles of the time (though there might be issues depending on the number of face buttons). This has since been phased out in favour of proprietary controller ports, or in the case of computers, has largely been replaced with the USB standard.
Video Game Systems that use DE-9-based Controllers