Serial communication has been used by computers for many years though it is not as widely used in today’s market. When most people refer to serial, they think of the DB9 cable.
It is referred to as DB9 because it carries 9 pins within it. There is also DB25 in the serial family too, which obviously carries 25 pins. DB9 was typically used for devices such as gamepads, mice, keyboards, or other such interface devices.
The StarTech NM9FF null modem adapter features two DB9 serial female connectors on either end to allow you to connect two male serial cables together. This is a great cost-saving way to convert a straight through cable into a null modem cable.
Null modems are usually used to connect DTE (data terminal equipment) like terminals or printers together using an RS-232 cable. Originally, the RS-232 standard was developed and used for teleprinter machines which could communicate with each other over phone lines. This was so that you wouldn’t have to use an actual modem for transferring of information between the two devices.