RS449 Basics, Interface and Pinout

- a summary or tutorial about the basics of RS449 data communications standard, what it is, the RS449 pinout and the RS 449 interface.

The RS449 or RS-449 interface is a further enhancement of RS232 and RS423. It is aimed at catering for very fast serial data communications at speeds up to 2 Mbps. In order to achieve this RS449 makes some changes when compared to RS232 to the way in which the signals are referenced, while still being able to retain some compatibility with RS232.

The RS499 standard which has now been discontinued is also known by the references EAI-449, TIA-449 and ISO 4902


RS449 interface

One of the ways in which the RS449 data communications standard is able to send at high speeds without stray noise causing interference is to use a differential form of signalling. Earlier data communications standards such as RS232 used signalling that was referenced to earth and while this was easier to implement and cheaper to cable, it introduced limitations into the system.

By using twisted wire pairs for the data lines, any unwanted noise will be picked up by both wires together. As the RS449 receivers use a differential input, and they are not referenced to ground, any noise that is picked up does not affect the input. This means that higher levels of noise can be tolerated without any degradation to the performance to the data communications system.

For the RS449 interface, ten additional circuits functions have been provided when compared to RS232. Additionally three of the original interchange circuits have been abandoned.

In order to minimise any confusion that could easily occur, the circuit abbreviations have been changed. In addition to this the RS449 interface requires the use of 37 way D-type connectors and 9 way D-type connectors, the latter being necessary when use is made of the secondary channel interchange circuits.


RS449 Primary connector pinout and interface connections

The RS449 primary connector, which is used the one that is used as standard uses a 37 way D-type connector. The pinout and connections are given in the table below:


Pin Signal Name Description
1   Shield
2 SI Signal Rate Indicator
3 n/a unused
4 SD- Send Data (A)
5 ST- Send Timing (A)
6 RD- Receive Data (A)
7 RS- Request To Send (A)
8 RT- Receive Timing (A)
9 CS- Clear To Send (A)
10 LL Local Loopback
11 DM- Data Mode (A)
12 TR- Terminal Ready (A)
13 RR- Receiver Ready (A)
14 RL Remote Loopback
15 IC Incoming Call
16 SF/SR+ Signal Freq./Sig. Rate Select.
17 TT- Terminal Timing (A)
18 TM- Test Mode (A)
19 SG Signal Ground
20 RC Receive Common
21 n/a unused
22 SD+ Send Data (B)
23 ST+ Send Timing (B)
24 RD+ Receive Data (B)
25 RS+ Request To Send (B)
26 RT+ Receive Timing (B)
27 CS+ Clear To Send (B)
28 IS Terminal In Service
29 DM+ Data Mode (B)
30 TR+ Terminal Ready (B)
31 RR+ Receiver Ready (B)
32 SS Select Standby
33 SQ Signal Quality
34 NS New Signal
35 TT+ Terminal Timing (B)
36 SB Standby Indicator
37 SC Send Common

RS449 primary connector pinout and connections

Within the RS449 interface a number of differential connections are defined. In the pinout table above they are labelled as either "A and B" or "+" and "-". When setting up a connection, it is necessary to ensure that the correct polarities are used. As twisted pairs are used for the A and B connections it is often possible to mix the. If this happens the interface will not work.


RS449 auxilliary connector

A second connector is defined for use when the secondary channel interchange circuits are needed.. This connector uses a 9 way D-type connector.


Pin Signal Name Description
1   Shield
2 SRR Secondary Receive Ready
3 SSD Secondary Send Data
4 SRD Secondary Receive Data
5 SG Signal Ground
6 RC Receive Common
7 SRS Secondary Request to Send
8 SCS Secondary Clear to Send
9 SC Send Common

RS449 secondary connector

The RS449 data communications interface is an interface standard that is able to provide data communications with speeds of up to 2 Mbps. Retaining some similarities to RS232, it is a more comprehensive interface capable of greater speeds and operation with greater levels of data integrity.

By Ian Poole


Read more popular data communications tutorials . . . . .

Ethernet RS-232 RS-422 RS-449
RS-485 Serial data comms USB Current loop

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