D-sub connectors are a common interface for RS-232 serial interfaces. The two most common or standard connectors are the 9-pin and then 25-pin. The 9-pin is the most widely used due to there being very few applications with require more than nine wires. The 25-pin connector contains wires to maintain two simultaneous communication channels however very few devices ever made use of this capability. There are 26-pin and 6-pin connectors for specific applications but these were also very seldom used.
25-pin to 9-pin Link
A 25-pin to 9-pin link is sometimes necessary depending upon the interfaces of the device which are to communicate with each other. In this configuration, the corresponding wire in the 25-pin connector is connected to the related wire in the 9-pin connector.
A gender changer is an adapter than can change a male interface into a female interface or a female interface into a male interface. In a 9-pin to 9-pin gender change the corresponding from each connector wires are connected to each other. A gender change and 25-pin to 9-pin link can be facilitated at the same time simple by connecting the corresponding wires in a direct method rather than crossing any signals.
Original serial communication was designed such that DTE(data terminal equipment) would interact with DCE(data communication equipment). However, often times there is a desire for DTEs (computers) to communicate directly to one and another without the use of DCE( modems ). Null modem cables can be configured using various specifications depending on the hardware, software and application for which the cable is intended.