| Two types of serial devices are available: Data Terminal Equipment and Data Communication Equipment. DTE devices are normally the source or destination devices, while DCE devices normally connect the source and destination together. Common examples of DTE equipment are terminals and printers, while the most common example of a DCE device is a modem. The main difference between the two, as far as we are concerned, is that they transmit and receive data on different pins, as shown in Table 14.2. Table 14.2. DTE/DCE Transmit and Receive DetailsHardware Type | Transmit | Receive |
|---|
DTE | 2 | 3 | DCE | 3 | 2 |
The RS-232 specification is titled "Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange," which shows it was originally designed to connect DTEs together by adding a DCE between them. It is, however, common practice to connect DTEs without the intermediate DCEwe just need to reconfigure the cable connections. DTEDCE Connection This is used to connect a computer to a modem. It is the designated type of connection and, as such, allows a "straight-through" cable to be easily used by connecting pin 2 (transmit) of the DTE device to pin 2 (receive) of the DCE device. A similar connection is made for transmit and receive connections on pin 3. Figure 14.3 shows the typical cable pin-outs we would use for this type of connection. Figure 14.3. DTEDCE (modem) cable. 
DTEDTE Connection This type of connection is used to connect a terminal or printer to a computer. For this to work, we alter some of the connections to account for the fact that both devices transmit on the same pin, and both receive on the same pin. We need to connect the transmit pin (2) on one device to the receive pin (3) on the other, and vice versain other words, cross pins 2 and 3. This is commonly known as a "null-modem cable." Although there are a number of different "recommended" ways to make these cables, we've shown two of the more-common ones in Figure 14.4. One is often used to connect terminals, while the second is used for other DTE devices that have full modem control support. Figure 14.4. DTEDTE (null-modem) cables. 
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