9 pin D-SUB male connector at the computer.
RS-449 (also known as EIA-449 and standardised mechanically by ISO 4902) is a general-purpose DTE-to-DCE serial interface introduced in 1977 that references RS-422 balanced signalling for high-speed circuits and RS-423 unbalanced signalling for lower-speed ones. In addition to the primary 37-position D-sub connector, RS-449 specifies an optional 9-position DE-9 secondary connector that carries auxiliary (secondary) channel circuits used for reverse-channel or diagnostic signalling. The standard was superseded by TIA/EIA-530-A in 1992, which provides the same functional circuits in the more compact DB-25 connector already common on RS-232 equipment, though RS-449 hardware remained in service on legacy telecom equipment for many years thereafter.
| Pin | Name | RS232 | V.24 | Dir | Description |
|---|---|---|---|---|---|
| 1 | n/a | 101 | - | Shield | |
| 2 | SSR | SRR | 122 | IN | Secondary Receiver Ready |
| 3 | SSD | SSD | 118 | OUT | Secondary Send Data |
| 4 | SRD | SRD | 119 | IN | Secondary Receive Data |
| 5 | SG | SG | 102 | - | Signal Ground |
| 6 | RC | RC | 102b | - | Receive Common |
| 7 | SRS | SRS | 120 | OUT | Secondary Request To Send |
| 8 | SCS | SCS | 121 | IN | Secondary Clear To Send |
| 9 | SC | SC | 102a | - | Send Common |
Signals
- Shield — Cable shield / chassis ground (V.24 circuit 101); not an active signal.
- SSR — Secondary Receiver Ready (V.24 circuit 122); IN from DCE; indicates DCE secondary channel is ready to receive.
- SSD — Secondary Send Data (V.24 circuit 118); OUT from DTE; secondary channel transmitted data.
- SRD — Secondary Receive Data (V.24 circuit 119); IN from DCE; secondary channel received data.
- SG — Signal Ground (V.24 circuit 102); common voltage reference for all unbalanced circuits.
- RC — Receive Common (V.24 circuit 102b); DCE-side common return for balanced receiver circuits; used with RS-422 differential pairs.
- SRS — Secondary Request To Send (V.24 circuit 120); OUT from DTE; DTE requests permission to transmit on secondary channel.
- SCS — Secondary Clear To Send (V.24 circuit 121); IN from DCE; DCE grants secondary channel transmission.
- SC — Send Common (V.24 circuit 102a); DTE-side common return for balanced transmitter circuits; used with RS-422 differential pairs.
Notes
- Direction is DTE (computer) relative to DCE (modem).
- The RS232 column gives the RS-232 circuit name; the V.24 column gives the corresponding ITU-T V.24 circuit number.
- The secondary connector is optional; not all RS-449 equipment implements it. When present it carries the auxiliary channel, which typically operates at a lower data rate than the primary channel and is used for reverse-channel signalling or out-of-band diagnostics.
- Circuits SG (pin 5), RC (pin 6), and SC (pin 9) provide signal ground and the two common-return paths required by RS-422 balanced signalling. RC is the receive-side return; SC is the send-side return.
- RS-449 references RS-422 (TIA/EIA-422-B) balanced differential signalling for data and timing circuits, giving a maximum data rate of 10 Mbit/s and cable lengths up to approximately 1200 m at lower rates.
- The 9-pin secondary connector is mechanically identical to the DE-9 shell used in RS-232 applications, though the pinout and electrical characteristics differ.
References
- AN-1031: TIA/EIA-422-B Overview (Rev. B) — Texas Instruments; RS-422 balanced differential specifications, data rate and cable length limits.
- AN-917: Popular Connector Pin Assignments for Data Communications — Texas Instruments; RS-449 DB-37 primary and DE-9 secondary connector pin assignment reference.
- ITU-T Recommendation V.24: List of Definitions for Interchange Circuits Between DTE and DCE — ITU-T; defines the V.24 circuit numbers referenced in the pin table (circuits 101, 102, 102a, 102b, 118–122).
- Overview of Serial Communication Standards — Sealevel Systems; comparative overview of RS-449, RS-422, RS-530, and related standards.