The RS232-C interface was developed for a single purpose. This purpose is stated by its title:
“Interface Between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange.”
Every word in the title is significant: it describes the interface between a terminal (DTE) to a modem (DCE) for the transfer of serial data.
Short for Recommended Standard-232C, a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices. In 1987, the EIA released a new version of the standard and changed the name to EIA-232-D. And in 1991, the EIA teamed up with Telecommunications Industry Association (TIA) and issued a new version of the standard called EIA/TIA-232-E. Many people, however, still refer to the standard as RS232C or just RS232.
Almost all modems conform to the EIA-232 standard and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to an EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).
The EIA-232 standard supports two types of connectors, a 25-pin D-sub type connector (DB25) and a 9-pin D-sub type connector (DB9). The type of serial communications used by PCs requires only 9 pins so either type of connector will work equally well.
Although EIA-232 is still the most common standard for serial communication, the EIA has recently defined successors to EIA-232 called RS422 and RS423. The new standards are backward compatible so that RS232 devices can connect to an RS422 port.