DCE to DTE serial connections

DCE equipment is typically a modem or other type of communication device. The DCE sits between the DTE (data terminal equipment) and a transmission circuit such as a phone line. Originally, the DTE was a dumb terminal or printer, but today it is a computer, or a bridge or router that interconnects local area networks. In an IBM mainframe environment, a communication controller and a link-attached cluster controller are examples of DTEs.

A DCE provides a connection for the DTE into a communication network and back again. In addition, it terminates and provides clocking for a circuit. When analog telephone lines are the communication media, the DCE is a modem. When the lines are digital, the DCE is a CSU/DSU (channel service unit/data service unit).

DTE and DCE interfaces are defined by the physical layer in the OSI (Open Systems Interconnection) model. The most common standards for DTE/DCE devices are EIA (Electronic Industries Association) RS-232-C and RS-232-D. Outside the United States, these standards are the same as the V.24 standard of the CCITT (Consultative Committee for International Telegraphy and Telephony). Other DTE/DCE standards include the EIA RS-366-A, as well as the CCITT X.20, X.21, and V.35 standards. The later standards are used for high-speed communication over telephone lines.

DTE and DCE devices send and receive data on separate wires that terminate at a 25-pin connector. It is useful to know that DTE devices transmit on pin connector 2 and receive on pin 3. DCE devices are just the opposite-pin 3 transmits and pin 2 receives.

For pinouts see:
http://www.dracko.com/index.php?s=page&n=show_post&id=417

The serial pin designations were originally intended for connecting a dumb terminal to a modem. The terminal was DTE (Data Terminal Equipment) and the modem was DCE (Data Communication Equipment). Today the PC is usually used as DTE instead of a terminal (but real terminals may still be used this way). The names of the pins are the same on both DTE and DCE. The words: "receive" and "transmit" are from the "point of view" of the PC (DTE). The transmit pin from the PC transmits to the "transmit" pin of the modem (but actually the modem is receiving the data from this pin so from the point of view of the modem it would be a receive pin).

The serial port was originally intended to be used for connecting DTE to DCE which makes cabling simple: just use a straight-thru cable. Thus when one connects a modem one seldom needs to worry about which pin is which. But people wanted to connect DTE to DTE (for example a computer to a terminal) and various ways were found to do this by fabricating various types of special null-modem cables. In this case what pin connects to what pin becomes significant.

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