DTE & DCE

Differentiating The Two Sides Of An RS-232 Circuit

The two sides of an RS-232 circuit are referred to as the DTE and DCE (both acronyms are pronounced by saying the three letters.) DTE means Data Terminal Equipment and DCE means Data Communications Equipment. The importance of differentiating between a DTE and a DCE is that pin-outs for cables and descriptions of handshaking and interfacing refer to the "DTE side" and the "DCE side" of a connection.

Unless special cable configurations are used, a DTE can only talk to a DCE, and vice versa. The origin of the terms will help make them clear (because today the terms are in use but the meaning tends to get "gray" sometimes!).

Here we see a user sitting at a computer, talking to another computer across an internetwork. The users computer (terminal) is connected to an interconnect device (perhaps a terminal server) using an RS-232 connection. The interconnect device in the figure is using some type of Wide Area Network interconnectivity to talk to a remote device. Perhaps the interconnect device is an X.25 Packet Assembler/Disassembler (PAD). The remote interconnect device is then using an Ethernet network to communicate to a host computer. Notice that RS-232 plays only one part of the end-to-end connectivity process. The user's device is the DTE and the interconnect device is the DCE.

The terms 'DTE' and 'DCE' are used in describing communications other than RS-232. Be very sure that you always compare like definitions. That is, don't start thinking about the DTE and DCE in an X.25 network while you are applying the concepts of DTE/DCE from RS-232. In this discussion we are using the terms strictly as they are defined by RS-232.

Some common, typical DTE's and DCE's are listed below to help you differentiate between them and remember what they do.

These are typical DCE's

• Modem
• X.25 PAD
• Terminal server

These are typical DTE's

• Desktop computer
• Notebook computer
• ASCII terminal
• Printer
• Router console port

From the table, you can see that it is also common for two DTE's to be connected directly together. When a terminal is attached to the router (for the purposed of router configuration) you've got a DTE-to-DTE connection. In fact, when a terminal is connected directly to a host computer, it's usually DTE-to-DTE.

In this example there are two DTE's connected together. This requires a special cable called a null modem cable. The cable takes the place of the normal signal correlation that is present when a DTE talks to a DCE (this will be discussed in great detail in just a few pages). A computer running a terminal program (perhaps accessing the Internet) is a very common DTE. An external modem is a very common DCE. (An internal modem is, technically, not a DCE because DCE and DTE are terms that define the two ends of an RS-232 connection. The internal modem is bus-attached, not RS-232 attached).