What is a D-Sub port?

D-Sub and DVI: standard interfaces for PC use

D-Sub and DVI are the current standard interfaces in PC environments.

Known officially as D-Sub miniature, D-Sub is not exclusive to display use. It's also used for serial-port, parallel-port, SCSI, and other connectors, with the number of connector pins depending on the purpose of use. However, these connector standards are rarely if ever found in PCs now for general-purpose personal use, most such applications having migrated to USB.

When used as a monitor interface, a D-Sub port is also known as a VGA port, an analog connection standard that's been around for some time. The connector is a DE-15 connector with 15 pins in three rows, often referred to as a "mini-D-Sub 15-pin" or "D-Sub 15-pin" connector. (Some connectors omit unused pins.) D-Sub is currently the most widely used monitor interface, compatible with very large numbers of PCs and LCD monitors.

A D-Sub female connector (photo at left) installed on the monitor side and  a D-Sub male connector (center photo) on the cable side. A D-Sub cable  features a screw on each end of the connector that can be turned by hand  to prevent unintended disconnection (photo at right).

The Digital Visual Interface (DVI) standard uses one of three types of connectors: DVI-D for digital connection; DVI-A for analog connection; and DVI-I, compatible with both digital and analog connections. The DVI-A connector for analog use is not in general use and can be disregarded when choosing monitor products.

Keep in mind that there are two types of mainstream DVI-D digital connections: single link and dual link. For a single-link DVI-D connection, the maximum resolution that can be displayed is 1920 × 1200 pixels (WUXGA). Higher resolutions (such as 2560 × 1600 pixels) require a dual-link DVI-D connection providing double the bandwidth of a single-link DVI-D (7.4 Gb/second or higher). To use a dual-link DVI-D connection, the DVI-D input on the LCD monitor side, the DVI-D output on the PC side, and the DVI-D cable must all be compatible with the dual-link DVI-D standard.

DVI-I, the other DVI standard, can be used with both digital and analog connections, depending on the monitor cable used. Since a DVI-I analog signal is compatible with the D-Sub standard, an analog connection can be formed by using a monitor cable with a D-Sub connector on one end and a DVI-I connector on the other. Depending on the cable and the connectors on the PC side and on the LCD-monitor side, it may also be possible to use an adapter for connecting a DVI-I connector with a D-Sub connector.

A DVI-D female connector installed on the monitor side (photo at left)  and a DVI-D single-link (18-pin) male connector installed on the cable  (center photo). As with D-Sub cables, a DVI-D cable can be secured into  place by turning the screws on either end of the connector (photo at right).
Pin layouts identify the DVI connector type. At left is a DVI-D dual-link (24-pin) connector; at right is a DVI-A (17-pin) connector.
At left is a DVI-I single-link (23-pin) connector; at right is a DVI-I dual-link (29-pin) connector.

Monitor cables with DVI-I connectors on both ends were available at one time. These are rare today, since this configuration made it difficult to determine whether the connection was digital or analog and generated frequent connection issues. Having DVI-I connectors on both the PC side and the LCD monitor side can lead to confusion. In such cases, the ideal configuration is a digital connection made with a DVI-D cable.

Ref: https://www.eizoglobal.com/library/basics/displayport_to_d-sub/#:~:text=When%20used%20as%20a%20monitor,Some%20connectors%20omit%20unused%20pins.)