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.
Primary channel
| Pin | Name | V.24 | Dir | Description | Type |
|---|---|---|---|---|---|
| 1 | 101 |  | Shield | Ground | |
| 2 | SI | 112 |  | Signal Rate Indicator | Control |
| 3 | n/a | n/a | unused | ||
| 4 | SD- | 103 | | Send Data (A) | Data |
| 5 | ST- | 114 |  | Send Timing (A) | Timing |
| 6 | RD- | 104 | | Receive Data (A) | Data |
| 7 | RS- | 105 | | Request To Send (A) | Control |
| 8 | RT- | 115 | | Receive Timing (A) | Timing |
| 9 | CS- | 106 | | Clear To Send (A) | Control |
| 10 | LL | 141 | | Local Loopback | Control |
| 11 | DM- | 107 | | Data Mode (A) | Control |
| 12 | TR- | 108.2 | | Terminal Ready (A) | Control |
| 13 | RR- | 109 | | Receiver Ready (A) | Control |
| 14 | RL | 140 | | Remote Loopback | Control |
| 15 | IC | 125 | | Incoming Call | Control |
| 16 | SF/SR+ | 126 |  | Signal Freq./Sig. Rate Select. | Control |
| 17 | TT- | 113 | | Terminal Timing (A) | Timing |
| 18 | TM- | 142 | | Test Mode (A) | Control |
| 19 | SG | 102 | | Signal Ground | Ground |
| 20 | RC | 102b | | Receive Common | Ground |
| 21 | n/a | n/a | unused | ||
| 22 | SD+ | 103 | | Send Data (B) | Data |
| 23 | ST+ | 114 | | Send Timing (B) | Timing |
| 24 | RD+ | 104 | | Receive Data (B) | Data |
| 25 | RS+ | 105 | | Request To Send (B) | Control |
| 26 | RT+ | 115 | | Receive Timing (B) | Timing |
| 27 | CS+ | 106 | | Clear To Send (B) | Control |
| 28 | IS | n/a | | Terminal In Service | Control |
| 29 | DM+ | 107 | | Data Mode (B) | Control |
| 30 | TR+ | 108.2 | | Terminal Ready (B) | Control |
| 31 | RR+ | 109 | | Receiver Ready (B) | Control |
| 32 | SS | 116 | | Select Standby | Control |
| 33 | SQ | 110 | | Signal Quality | Control |
| 34 | NS | n/a | | New Signal | Control |
| 35 | TT+ | 113 | | Terminal Timing (B) | Timing |
| 36 | SB | 117 | | Standby Indicator | Control |
| 37 | SC | 102a | | Send Common | Ground |
Note: Direction is DTE (Computer) relative DCE (Modem).
| Name | Description | Function |
|---|---|---|
| AA | Shield Ground | Also known as protective ground. This is the chassis ground connection between DTE and DCE. |
| AB | Signal Ground | The reference ground between a DTE and a DCE. Has the value 0 Vdc. |
| BA | Transmitted Data | Data send by the DTE. |
| BB | Received Data | Data received by the DTE. |
| CA | Request To Send | Originated by the DTE to initiate transmission by the DCE. |
| CB | Clear To Send | Send by the DCE as a reply on the RTS after a delay in ms, which gives the DCEs enough time to energize their circuits and synchronize on basic modulation patterns. |
| CC | DCE Ready | Known as DSR. Originated by the DCE indicating that it is basically operating (power on, and in functional mode). |
| CD | DTE Ready | Known as DTR. Originated by the DTE to instruct the DCE to setup a connection. Actually it means that the DTE is up and running and ready to communicate. |
| CE | Ring Indicator | A signal from the DCE to the DTE that there is an incomming call (telephone is ringing). Only used on switched circuit connections. |
| CF | Received Line Signal Detector | Known as DCD. A signal send from DCE to its DTE to indicate that it has received a basic carrier signal from a (remote) DCE. |
| CH/CI | Data Signal Rate Select (DTE/DCE Source> | A control signal that can be used to change the transmission speed. |
| DA | Transmit Signal Element Timing (DTE Source) | Timing signals used by the DTE for transmission, where the clock is originated by the DTE and the DCE is the slave. |
| DB | Transmitter Signal Element Timing (DCE Source) | Timing signals used by the DTE for transmission. |
| DD | Receiver Signal Element Timing (DCE Source) | Timing signals used by the DTE when receiving data. |
| IS | terminal In Service | Signal that indicates that the DTE is available for operation |
| NS | New Signal | A control signal from the DTE to the DCE. It instructs the DCE to rapidly get ready to receive a new analog signal. It helps master-station modems rapidly synchronize on a new modem at a tributary station in multipoint circuits |
| RC | Receive Common | A signal return for receiver circuit reference |
| LL | Local Loopback / Quality Detector | A control signal from the DTE to the DCE that causes the analog transmision output to be connected to the analog receiver input. |
| RL | Remote Loopback | Signal from the DTE to the DCE. The local DCE then signals the remote DCE to loopback the analog signal and thus causing a line loopback. |
| SB | Standby Indicator | Signal from the DCE to indicate if it is uses the normal communication or standby channel |
| SC | Send Common | A return signal for transmitter circuit reference |
| SF | Select Frequency | A signal from the DTE to tell the DCE which of the two analog carrier frequencies should be used. |
| SS | Select Standby | A signal from DTE to DCE, to switch between normal communication or standby channel. |
| TM | Test Mode | A signal from the DCE to the DTE that it is in test-mode and can"t send any data. |
| Reserved for Testing |
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.
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