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
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).