37 PIN D-SUB MALE at the DTE (Computer)
37 PIN D-SUB FEMALE at the DCE (Modem)
The RS-449 (EIA-449, 1977) interface uses a 37-position D-sub connector and defines the mechanical and functional characteristics for serial DTE-to-DCE communication, employing TIA/EIA-422-B balanced differential signalling for its primary circuits. TIA/EIA-422-B specifies a minimum differential output of 2 V across a 100 Ω load, receiver sensitivity of ±200 mV, and supports data rates up to 10 Mbit/s over cable lengths of up to approximately 1200 m at lower speeds — though maximum rate and maximum distance cannot be achieved simultaneously. RS-449 was introduced to overcome the limitations of RS-232 by offering higher speed and better noise immunity through differential twisted-pair wiring; it was formally superseded by EIA/TIA-530-A in 1992, which delivered equivalent functionality via the more compact DB-25 connector, though RS-449 equipment remains in use on legacy telecommunications hardware.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | GND | - | Shield Ground |
| 2 | SRI | IN | Signal Rate Indicator |
| 3 | n/c | - | Spare |
| 4 | SD | OUT | Send Data |
| 5 | ST | OUT | Send Timing |
| 6 | RD | IN | Receive Data |
| 7 | RTS | OUT | Request To Send |
| 8 | RR | IN | Receiver Ready |
| 9 | CTS | IN | Clear To Send |
| 10 | LL | OUT | Local Loopback |
| 11 | DM | IN | Data Modem |
| 12 | TR | OUT | Terminal Ready |
| 13 | RR | IN | Receiver Ready |
| 14 | RL | OUT | Remote Loopback |
| 15 | IC | IN | Incoming Call |
| 16 | SF/SR | OUT | Select Frequency/Select Rate |
| 17 | TT | OUT | Terminal Timing |
| 18 | TM | IN | Test Mode |
| 19 | GND | - | Ground |
| 20 | RC | - | Receive Twister-Pair Common |
| 21 | GND | - | Spare Twister-Pair Return |
| 22 | /SD | - | Send Data TPR |
| 23 | GND | - | Send Timing TPR |
| 24 | GND | - | Receive Timing TPR |
| 25 | /RS | - | Request To Send TPR |
| 26 | /RT | - | Receive Timing TPR |
| 27 | /CS | - | Clear To Send TPR |
| 28 | IS | IN | Terminal In Service |
| 29 | /DM | - | Data Mode TPR |
| 30 | /TR | - | Terminal Ready TPR |
| 31 | /RR | - | Receiver TPR |
| 32 | SS | OUT | Select Standby |
| 33 | SQ | IN | Signal Quality |
| 34 | NS | OUT | New Signal |
| 35 | /TT | - | Terminal Timing TPR |
| 36 | SB | IN | Standby Indicator |
| 37 | SC | - | Send Twister Pair Common |
Notes
- Direction is DTE (Computer) relative to DCE (Modem).
- RS-449 uses TIA/EIA-422-B balanced differential signalling. Unlike RS-232, which references each signal to a common ground, RS-422 receivers detect the voltage difference between a twisted pair of wires (the A/+ and B/− conductors). Because both wires of a twisted pair pick up the same electromagnetic interference, that noise cancels at the differential receiver — giving RS-449 far better noise immunity and allowing much higher speeds and longer cable runs than RS-232.
- Differential signal pairs in RS-449 are labelled “A and B” or “+ and −”. Wire A (or +) must connect to wire A (or +) at the far end, and B (or −) to B (or −). Reversing the pair inverts the data or clock polarity; no damage results, but the interface will not function correctly.
- TIA/EIA-422-B common-mode voltage range at the receiver is −7 V to +7 V. Up to 10 receivers may be connected to a single driver in a simplex multidrop configuration.
- Cable must use twisted pairs for each differential signal group. Recommended characteristic impedance is 100 Ω; line termination resistors equal to the cable impedance are used on terminated circuits to suppress reflections at higher data rates.
- Pins labelled “TPR” in the table are the Twisted Pair Return conductors (the B/− side of each differential pair). The “/” prefix on a signal name (e.g. /SD, /RS) denotes the inverted (B) leg of that pair.
Speed and Distance
(Max Recommended)
Terminated Circuits
- 10 MHz 10 Meters
- 6 MHz 17 Meters
- 2 MHz 40 Meters
- 1 MHz 100 Meters
- 100 KHz 1000 Meters
- 10 KHz 1000 Meters
Non- Terminated Circuits
- 1 MHz 10 Meters
- 100 KHz 100 Meters
- 56 KHz 110 Meters
- 10 KHz 1000 Meters