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RS-449 (EIA-449) interface


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
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.

Primary channel

Pin Name V.24 Dir Description Type
1 101---ShieldGround
2SI112-->Signal Rate IndicatorControl
3n/a n/aunused 
4SD-103-->Send Data (A)Data
5ST-114<--Send Timing (A)Timing
6RD-104<--Receive Data (A)Data
7RS-105-->Request To Send (A)Control
8RT-115<--Receive Timing (A)Timing
9CS-106<--Clear To Send (A)Control
10LL141-->Local LoopbackControl
11DM-107<--Data Mode (A)Control
12TR-108.2-->Terminal Ready (A)Control
13RR-109<--Receiver Ready (A)Control
14RL140-->Remote LoopbackControl
15IC125<--Incoming CallControl
16SF/SR+126<->Signal Freq./Sig. Rate Select.Control
17TT-113-->Terminal Timing (A)Timing
18TM-142<--Test Mode (A)Control
19SG102---Signal GroundGround
20RC102b---Receive CommonGround
21n/a n/aunused 
22SD+103-->Send Data (B)Data
23ST+114<--Send Timing (B)Timing
24RD+104<--Receive Data (B)Data
25RS+105-->Request To Send (B)Control
26RT+115<--Receive Timing (B)Timing
27CS+106<--Clear To Send (B)Control
28ISn/a-->Terminal In ServiceControl
29DM+107<--Data Mode (B)Control
30TR+108.2-->Terminal Ready (B)Control
31RR+109<--Receiver Ready (B)Control
32SS116<--Select StandbyControl
33SQ110<--Signal QualityControl
34NSn/a-->New SignalControl
35TT+113-->Terminal Timing (B)Timing
36SB117<--Standby IndicatorControl
37SC102a---Send CommonGround

Note: Direction is DTE (Computer) relative DCE (Modem).

Name Description Function
AAShield GroundAlso known as protective ground. This is the chassis ground connection between DTE and DCE.
ABSignal GroundThe reference ground between a DTE and a DCE. Has the value 0 Vdc.
BATransmitted DataData send by the DTE.
BBReceived DataData received by the DTE.
CARequest To SendOriginated by the DTE to initiate transmission by the DCE.
CBClear To SendSend 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.
CCDCE ReadyKnown as DSR. Originated by the DCE indicating that it is basically operating (power on, and in functional mode).
CDDTE ReadyKnown 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.
CERing IndicatorA signal from the DCE to the DTE that there is an incomming call (telephone is ringing). Only used on switched circuit connections.
CFReceived Line Signal DetectorKnown 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/CIData Signal Rate Select
(DTE/DCE Source>
A control signal that can be used to change the transmission speed.
DATransmit 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.
DBTransmitter Signal Element Timing
(DCE Source)
Timing signals used by the DTE for transmission.
DDReceiver Signal Element Timing
(DCE Source)
Timing signals used by the DTE when receiving data.
ISterminal In ServiceSignal that indicates that the DTE is available for operation
NSNew SignalA 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
RCReceive CommonA signal return for receiver circuit reference
LLLocal Loopback / Quality DetectorA control signal from the DTE to the DCE that causes the analog transmision output to be connected to the analog receiver input.
RLRemote LoopbackSignal 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.
SBStandby IndicatorSignal from the DCE to indicate if it is uses the normal communication or standby channel
SCSend CommonA return signal for transmitter circuit reference
SFSelect FrequencyA signal from the DTE to tell the DCE which of the two analog carrier frequencies should be used.
SSSelect StandbyA signal from DTE to DCE, to switch between normal communication or standby channel.
TMTest ModeA 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.

37 pin D-SUB female
37 pin D-SUB female connector  at the DCE (Modem)

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