EIA-449 (RS-449) interface

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).

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.