RS-232

Aside from a common data representation format, communication signaling such as framing or error detection also requires standardization so that equipment manufactured by different companies can exchange information. When one begins discussing communications, an unstoppable journey into the sometimes mysterious world of industry standards begins. Navigating these standards can be tricky because of subtle differences in terminology between related standards and the everyday jargon to which the engineering community has grown accustomed. Standards are living documents that are periodically updated, revised, or replaced. This shifting base of documentation can add other challenges to fully complying with a standard.
One of the most ubiquitous serial communications schemes in use is defined by the RS-232
family of standards. Most UARTs are designed specifically to support RS-232. Standards purists may balk at the common reference to RS-232 in the modern context, for several reasons. First, the original RS-232 document has long since been superseded by multiple revisions. Second, its name was changed first to EIA-232, then to EIA/TIA-232. And third, RS-232 is but one of a set of related standards that address asynchronous serial communications. These standards have been developed under the auspices of the Electronics Industry Alliance (formerly the Electronics Industry Association) and Telecommunications Industry Association. Technically, EIA/TIA-232 (first introduced in 1962 as RS-232) standardizes the 25-pin D-subminiature (DB25) connector and pin assignment along with an obsolete electrical specification that had limited range. EIA/TIA-423 standardizes the modern electrical characteristics that enable communication at speeds up to 100 kbps over short distances (10 m). EIA/TIA-574 standardizes the popular nine-pin DE9 connector that is used on most new “RS-232” equipped devices. These days, when most people talk about an RS-232 port, they are referring to the overall RS-232 family of related serial interfaces. In fairness to standards purists, this loose terminology is partially responsible for confusion among those who implement and use RS- 232. From a practical perspective, however, it is most common to use the term RS-232 with additional qualifiers (e.g., 9-pin or 25-pin) to convey your point. In fact, if you start mentioning EIA/ TIA-574 and 423, you will probably be met by blank stares from most engineers. This somewhat shady practice is continued here because of its widespread acceptance in industry.