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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