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Serial Data Transmission

Serial data transmission is the most common method of sending data from one DTE to another. During transmission, the data must pass through a serial interface to exit a computer as serial data. The following are common interfaces for serial data:

RS-232
Defines three types of connections: electrical, functional, and mechanical. The RS-232 interface is ideal for the data-transmission range of 0–20 kbps/50 ft. (15.2 m). It employs unbalanced signaling and is usually used with DB25 connectors to interconnect DTEs (computers, controllers, etc.) and DCEs (modems, converters, etc.). Serial data exits through an RS-232 port via the Transmit Data (TD) lead and arrives at the destination device’s RS-232 port through its Receive Data (RD) lead. RS-232 is compatible with these standards: ITU V.24, V.28; ISO IS2110.

RS-449
Defines functional/mechanical interfaces for DTEs/DCEs that employ serial binary data interchange and is usually used with synchronous transmissions. It identifies signals (TD, RD, etc.) that correspond with the pin numbers for a balanced interface on DB37 and DB9 connectors. RS-449 was originally intended to replace RS-232, but RS-232 and RS-449 are completely incompatible in mechanical and electrical specifications.

RS-530
Supersedes RS-449 and complements RS-232. Based on a 25-pin connection, it works in conjunction with either electrical interface RS-422 (balanced electrical circuits) or RS-423 (unbalanced electrical circuits).

RS-530 defines the mechanical/electrical interfaces between DTEs and DCEs that transmit serial binary data, sync or async, at rates from 20 kbps to 2 Mbps. (Maximum distance depends on the electrical interface.) RS-530 takes advantage of higher data rates with the same mechanical connector used for RS-232. Though RS-530 and RS-232 are not compatible, RS-530 is compatible with these standards: ITU V.10, V.11, X.26; MIL-188/114; RS-449.

RS-422
Defines a balanced interface with no accompanying physical connector. Manufacturers who adhere to this standard use many different connectors, including screw terminals, DB9, DB25 with nonstandard pinning, DB25 following RS-530, and DB37 following RS-449. RS-422 is commonly used in point-to-point communications conducted with a dual-state driver.

RS-485
Resembles RS-422. It may be used in multipoint applications where one computer controls many different devices. Up to 64 devices may be interconnected with RS-485.

V.35
Is the international standard termed ‘‘Data Transmission at 48 kbps Using 60–108 KHz Group-Band Circuits.’’ It is typically used for DTEs or DCEs that interface to a high-speed digital carrier such as AT&T® Dataphone® Digital Service (DDS).

NOTE: ITU (International Telegraphic Union) was formerly the CCITT.

 

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