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The RS-449 specification, also known as EIA-449 or TIA-449, defines the functional and mechanical characteristics of the interface between data terminal equipment, typically a computer, and data communications equipment, typically a modem or terminal server. It was an effort to replace RS-232C, offering much higher performance and longer cable lengths, but emerged as an unwieldy system requiring both DC-37 and DE-9 connectors. The effort was eventually abandoned in favor of RS-530, which used a single DB-25 connector. The full title of the standard is EIA-449 General Purpose 37-Position and 9-Position Interface for Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange.
During the late 1970s, the EIA began developing two new serial data standards to replace RS-232. RS-232 had a number of issues that limited its performance and practicality. Among these was the relatively large voltages used for signalling, +5 and -5V for mark and space. To supply these, a +12V power supply was typically required, which made it somewhat difficult to implement in a market that was rapidly being dominated by +5/0V transistor-transistor logic (TTL) circuitry and even lower-voltage CMOS implementations. These high voltages and unbalanced communications also resulted in relatively short cable lengths, nominally set to a maximum of 50 feet (15 m), although in practice they could be somewhat longer if running at slower speeds.[a]
The reason for the large voltages was due to ground voltages. RS-232 included both a protective ground and a signal ground in the standard, but did not define how these were to be implemented. It was often the case that the protective ground was left unconnected, and the signal ground was connected to ground at both ends. As a result, if there was a slight difference in ground potential at the two ends of the cable, the voltage in the signal ground pin might not be zero, and large signal voltages were needed to provide a positive signal in this case.
To address this problem, the new RS-422 and RS-423 standards used well-defined grounding that was always based on the sender's reference, and made the signal only 400 mV above or below this reference. In the case of RS-422, for instance, every signal had a second pin operating at the opposite voltage, thereby balancing the voltages and always providing a positive signal. When this process was starting, the decision was made to unbundle the mechanical aspects of the standard from the electrical, with the former becoming the RS-449 standards track.
The primary difference between RS-422 and RS-423 was that the former had a return line for every signal, while the later had a single shared signal ground. This meant that RS-422 had double the number of signal wires. Along with other changes, the number of connections began to grow, to the point where even RS-423, which was functionally similar to RS-232, no longer fit in a DB25 connector. This led to the use of the larger DC-37, but even that did not have enough pins to support RS-422, so this was "solved" by adding the additional ground wires to a separate DE-9 connector. This resulted in a "horrendous number of wires" and the conclusion in 1983 that its "success... remains to be seen."
The standard was rarely used, although it could be found on some network communication equipment. EIA-449-1 was rescinded in January 1986 and superseded by EIA/TIA-530-A, the final version EIA-449-1 was withdrawn in September 2002. The most widespread use of RS-422/423, the early Apple Macintosh computers, used a simple 9-pin DIN connector and for inter-machine links used only three-wire connectors.
- The need for 12V to drive RS-232 often led to some complexity in early computer power supplies. For instance, in the Atari 8-bit family, expansion devices were located on an external port, so the power supply had to feed both 5 and 12V to the machine, and then pass the 12V out though the SIO connector.
- Electronic Industries Association, EIA Standard 449 November 1977, reprinted in Telebyte Technology "Data Communication Library" Greenlawn NY, 1985, no ISBN, no Library of Congress card number
- Witten, Ian (February 1983). "Welcome to the Standards Jungle". Byte. Vol. 8 no. 2. pp. 146–178.