RS-449 was introduced in 1977 as an eventual replacement for RS-232-C. This interface specification calls for the use of a 37-pin connector as well as an optional 9-pin connector for devices using a secondary channel.

Unlike RS-232, RS-449 does not specify voltage levels. Two additional specifications known as RS-442-A and RS-423 cover voltage levels for a specific range of data speeds. RS-442-A and its counterpart, the ITU-T X.27 (V.11), define the voltage levels for data rates from20 kbps to 10 Mbps; whereas RS-423-A and its ITU-T counterpart, X.26 (V.10), define the voltage levels for data rates between 0 and 20 kbps.

RS-442 (as well as its ITU-T counterpart) defines the use of differential balanced signaling. RS-422 is designed for twisted-pair telephone wire transmission ranging from 10 Mbps at distances up to 40 feet to 100 kbps at distances up to 4000 feet. RS-423 defines the use of unbalanced signaling similar to RS-232. This standard supports data rates ranging from100 kbps at distances up to 40 feet, to 10 kbps at distances up to 200 feet.

The use of RS-422, RS-423 and RS-449 permits the cable distance between DTEs and DCEs to be extended to 4000 feet in comparison to RS-232’s 50 foot limitation. In comparing RS-449 to RS-232, you will note the addition of ten circuits which are either new control or status indicators and the deletion of three functions formerly provided by RS-232.

The most significant functions added by RS-449 are local and remote loopback signals. These circuits enable the operation of diagnostic features built into communications equipment via DTE control, permitting as an example, the loopback of the device to the DTE and its placement into a test mode of operation.

With the introduction of RS-232-D a local loopback function was supported. Thus, the column labeled RS-232 Destination with the row entry Local Loopback indicates that that circuit is only applicable to revisions D and E of that standard by the entries D/E in parentheses after the circuit name.

Although a considerable number of articles have been written describing the use of RS-449, its complexity has served as a constraint in implementing this standard by communications equipment vendors. Other constraints limiting its acceptance include the cost and size of the 37-pin connector arrangement and the necessity of using another connector for secondary operations.

By late 1999, less than a few percent of all communications devices were designed to operate with this interface. Due to the failure of RS-449 to obtain commercial acceptance, the EIA issued RS-530 in March, 1987. This new standard is intended to gradually replace RS-449.

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