DB9

DB9, also written as DB-9 or DE-9 in some references, is a 9-pin D-subminiature connector used primarily for RS-232 serial communications. It is part of the D-sub family of connectors, which feature a D-shaped metal shell and a particular arrangement of pins or sockets. The DB9 form has two rows of contacts—five on the top row and four on the bottom—and is commonly encountered as a male plug or female receptacle depending on the mating part.

The name DB9 reflects the shell size and pin count, though DE-9 is a close variant used in some contexts. The shell dimension is shared with other D-sub connectors in the family, but the 9-pin configuration distinguishes it from larger variants such as 15, 25, or more pins. The connector is widely recognized for its role in legacy computer serial ports and various industrial devices.

Pinout for RS-232 usage (DE-9): 1 DCD, 2 RxD, 3 TxD, 4 DTR, 5 GND, 6 DSR, 7 RTS, 8 CTS, 9 RI. Among these, pins 2 (RxD), 3 (TxD), and 5 (GND) carry the essential data and ground, while the remaining pins support signal and control lines such as flow control and status. Exact functions can vary by device and cable, and some deployments use only a subset of signals.

Applications and compatibility: DB9 connectors were standard on many early personal computers for serial ports and on a range of networking and industrial equipment. Today they are still used on some legacy systems, embedded equipment, and console interfaces on certain routers and switches, often connected via USB-to-serial adapters or other interface converters. See also D-sub connectors, RS-232, serial communication, and null-modem cables.