What Is a D-Sub Connector?

A D-sub connector is a multi-pin connector whose metal shell is shaped like a capital D, so the plug can only go in one way round. The full name is D-subminiature. ITT Cannon introduced it in the early 1950s, when it was small by the standards of the day, and it is now covered by the US military specification MIL-DTL-24308 and matching IEC standards.

Inside the shell the contacts sit in two or more staggered rows. The male half carries pins and the female half carries sockets. The shell grounds the cable shield to the equipment chassis, and two jackscrews, or a slide latch on older equipment, hold the plug in place. That screw retention is a large part of why D-sub is still fitted to industrial computers: a COM port cable fastened with jackscrews does not fall out when a cabinet door is slammed.

The shell size is given by a letter and the pin count by a number. A DB-25 is a B-size shell with 25 pins, and a DE-9 is an E-size shell with nine pins.

There are five standard shell sizes, plus high-density versions that fit an extra row of thinner contacts into the same shell. The same shell can also carry mixed layouts with larger power or coax contacts.

“DB9” and “DB15” are everyday names rather than strict ones. A nine-pin serial connector is correctly a DE-9, and the three-row VGA connector is a DE-15. Catalogs and datasheets use both, so check the pin count and row count rather than the name.

The male connector has exposed pins and the female has sockets. On a PC, convention puts a male DE-9 on serial COM ports, a female DE-15 on VGA outputs and a female DB-25 on parallel ports. Cables are usually built to match, with a gender changer or a null modem adapter used to join two connectors of the same gender.

Contacts are made in several termination styles: solder cup for hand-built cables, crimp for production harnesses, insulation displacement for ribbon cable, and straight or right-angle PCB pins for board mounting.

Pins are numbered from the top row, left to right when you look at the face of the male connector, and right to left on the female. Pin 1 is usually marked on the insulator. Getting this mirror image wrong is the most common mistake when wiring a D-sub by hand, so check the pinout against the face you are looking at before soldering, and test the finished cable with a continuity tester before it goes into a cabinet.

D-sub has gone from consumer PCs, but it remains common on industrial and embedded equipment, where machines and instruments last for decades.

A standard D-sub is not sealed, so it suits clean cabinets and enclosures rather than washdown or outdoor use. Where a serial or I/O connection has to survive water, dust or vibration, the usual options are IP67-rated D-sub connectors with a gasket and sealed backshell, or a switch to circular connectors such as M12. Metal backshells with strain relief and fully tightened jackscrews are the simplest improvement on any D-sub cable that sits on a machine.

For long or electrically noisy serial runs, the connector matters less than the interface. Changing from RS-232 to RS-485 on the same DE-9 typically extends reach to hundreds of meters.

We build industrial computers with the COM ports, serial cards and breakout cables the installation needs as standard, and set the RS-232, RS-422 or RS-485 mode on each port before the system ships. Send us your device list and we will specify the ports and cables to match.