The RS449 serial data standard was intended as an enhancement to RS232. It was aimed at providing serial data transmission at speeds up to 2 Mbps whilst still being able to maintain compatibility with RS232.
Though never applied on personal computers, this interface was found on some network communication equipment. The RS449 standard has now been discontinued and may also be seen in some references as EIA-449, TIA-449 and ISO 4902
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:
|2||SI||Signal Rate Indicator|
|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)|
|11||DM-||Data Mode (A)|
|12||TR-||Terminal Ready (A)|
|13||RR-||Receiver Ready (A)|
|16||SF/SR+||Signal Freq./Sig. Rate Select.|
|17||TT-||Terminal Timing (A)|
|18||TM-||Test Mode (A)|
|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)|
|35||TT+||Terminal Timing (B)|
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.
|2||SRR||Secondary Receive Ready|
|3||SSD||Secondary Send Data|
|4||SRD||Secondary Receive Data|
|7||SRS||Secondary Request to Send|
|8||SCS||Secondary Clear to Send|
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.
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