This type of connector has been used on systems built by Digital Equipment Corporation in the early days one of the leaders in the mainframe world. The pinout is also shown for the DEC modified modular jack. Note, that the protective ground is assigned to a pin at the large connector where the connector outside is used for that purpose with the DB9 connector version. The defined pins only present on the larger connector are shown in red. The diagrams show the signals common to both connector types in black.

On personal computers, the smaller DB9 version is more commonly used today. It can be used to query the modem status while the modem is on-line and busy communicating. Also on a number of Telebit modem models the secondary channel is present. Examples of this are the Sun SparcStation 10 and 20 models and the Dec Alpha Multia. Only very few computers have been manufactured where both serial RS232 channels are implemented. In practice, only one serial communication channel with accompanying handshaking is present. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. The RS232 connector was originally developed to use 25 pins. These pages are intended to provide information about the most common serial RS232 cables in normal computer use, or in more common language “How do I connect devices and computers using RS232?” RS232 serial connector pin assignment Almost nothing in computer interfacing is more confusing than selecting the right RS232 serial cable.

Db9 female serial port pinout diagram