The RS-232C (Recommended Standard-232C) standard is a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices. The standard defines two types of serial devices. The first of which is the DTE, or Data Terminal Equipment. DTE devices are responsible for processing data to be received or sent via a serial connection. Simply put, a DTE device is a PC with a UART chip. The second type of serial device is the DCE, or Data Communications Equipment. DCE devices communicate with DTE devices. A good example of the DCE device is a modem.
The RS-232C standard also defines how DTE and DCE devices are connected. It says that a DTE device will use a 25-pin D-type male connector, also known as a DB-25 male connector. DCE devices must use a DB-25 female connector. However modern PCs use a DB-9 male connector on the PC, and a DB-9 female connector on the DCE device.
RS-232C also defines that a DTE device (PC) is then connected to the DCE device using a straight through cable. This means that the pins used for transmitting data in a DTE device are connected to the pins responsible for receiving data in the DCE device. The standard also limits the length of this cable to 50 ft. Of course this limit can be exceeded depending on the quality of the cable that is used and the speed of transmission. If you use a high quality, well-shielded cable, lengths of well over a 1,000 ft. can be used. Of course, the higher the rate of transmission, the shorter your cable can be.
If you wish to connect two DTE devices, say two PCs, you must use a special type of cable called a null modem cable. A null modem cable is nothing more than a standard serial cable that has its receiving and transmit wires reversed. They can be purchased in any office supply or computer supply store for less than $10.00. By connecting two PCs with the null modem cable, you can perform some basic networking operations, such as file transfers.
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