The RS449 interface is a generic connector specification. It´s not an actual interface. The connector pinning was originally designed to support RS422 for balanced signals, and RS423 for the unbalanced signals. And should have been the succesor of RS232.
37 pin D-SUB male connector at the DTE (Computer)
RS449 is a high speed digital interface - unlike RS232 which uses signals with reference to ground, RS449 V.11 receivers look for the difference between two wires. By twisting the two wires and making a "twisted pair" any stray noise picked up on one wire will be picked up on the other, because both wires pick up the same noise the RS449 differential interface just shifts in voltage level with reference to ground, but does not change with respect to each other. The receivers are only looking at the difference in voltage level of each wire to the other not to ground.
The differential signals for RS449 are labeled as either "A and B" or "+ and -". In the case of RS449 wire A or + does not connect to B or -. Wire A always connects to A and B connects to B or + to + and - to -. If you do cross the wires you just inverted the data or clock in your interface and they don"t work - be sure to check the polarities .
Common names: EIA-449, RS-449, ISO 4902.
Note: Direction is DTE (Computer) relative DCE (Modem).
The EIA RS449 standard specifies the functional and mechanical characteristics of the RS449 interconnection between the data terminal equipment (DTE) in the data communications equipment (DCE) complying to EIA electrical interface standards RS 422 and RS 423.
37 pin D-SUB female connector at the DCE (Modem)