In 1973 the RS-232-C standard needed a major revision because of the accelerating rate of technological change. Three new standards incorporate new interface technology designed to overcome the shortcoming of RS-232-C.

The old standard had some problems:

• The data transmission rate was limited to 20 kbps.
• The distance for transmission is limited to 50 ft.
• The standard does not specify a connector, which led to some 25-pin designs not compatible with each other.
• Only one conductor per circuit is used, with only one signal return for both directions of transmission.
• The interface uses unbalanced transmitters and receivers (unbalanced circuit is less desirable (relative to performance) than a balanced circuit).
• The interface can generate considerable crosstalk among its component signals.
• The overall interface design is for discrete component technology.

The new standards had to achieve the following goals:

• Maintain compatibility with the old RS-232-C.
• Support a higher signaling rate bandwidth over longer distances than the RS-232-C.
• Add interface circuit to perform such functions as loop-back testing.
• Resolve the mechanical interface problems that were caused from the lack of connector specifications in the old standard.
• Improve the electrical characteristics of the interface by providing for balanced circuits.

The RS-422 standard defines a double-ended electrical interface module that can signal at rates well in excess of the 20 kbps limitation of RS-232-C. The mechanical connections for this interface are provided by the RS-449 and covered in its standard.

The single-ended electrical standard RS-423 is intended to reach the compatibility with RS-232-C, while simultaneously conforming to both RS-422-type electrical conventions and the RS-449 mechanical standard. The RS-423 is designed to connect to RS-232-C and RS-422 and its electrical specification is almost identical to RS-232-C. The RS-422 has balanced transmission and it is more reliable and has high-speed signaling rates.

TEST MODE is a new signal for RS-449, which in conjunction with other optional signals provides a means for testing the communications equipment.

From the standpoint of performance, the new standards are a distinct improvement , but considerations of cost and convenience, and general resistance of the user community to change, mean that some time will pass before theses standards become widespread.