There are usually two types of computer cases, and a new computer case may have only one. Laptops may not have any.
Many industrial instruments use it as a standard communication port. The content and format of communication are generally attached in the user manual of the instrument.
Data transmission between computers and computers or between computers and terminals can be carried out in two ways: serial communication and parallel communication. Due to the serial communication method having fewer lines used, lower cost, and especially in remote transmission, it avoids the inconsistency of multiple lines and is widely adopted. During serial communication, both communication parties are required to adopt a standard interface, enabling different devices to be conveniently connected for communication. The RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It was jointly developed by the American Electronic Industries Association (EIA), Bell System, modem manufacturers, and computer terminal manufacturers in 1970 for serial communication standards. Its full name is “Serial Binary Data Exchange Interface Standard between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)”. This standard stipulates the use of a 25-pin DB25 connector, specifying the signal content of each pin of the connector, and also stipulating the levels of various signals.