Data Terminal Ready (DTR) - Computer tells the modem that it is ready to talk. Transmit Data - Computer sends information to the modem. Receive Data - Computer receives information sent from the modem. Carrier Detect - Determines if the modem is connected to a working phone line. The following list describes the function of each pin of a 9-pin serial port when a modem is connected. Originally, the primary use of a serial port was to connect a modem to the computer. The external connector for a serial port can be either 9 pins or 25 pins. The UART chip transforms the parallel output of the computer’s system bus into serial form for transmission through the serial port. Serial ports rely on the universal asynchronous receiver/transmitter (UART) to function properly. Using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time. Serial devices use different pins to receive and transmit data allowing full-duplex communication. Bidirectional communication allows each device to receive data as well as transmit it. After each byte of data, it sends a stop bit to signal that the byte is complete. Before each byte of data, a serial port sends a start bit. The disadvantage is that it takes eight times longer to transmit the data than it would if there were eight wires. The advantage is that a serial port requires only one wire to transmit the eight bits, while a parallel port requires eight. The serial port transmits the eight bits in a data byte one bit at a time. USB ports are only a few years old, and will likely replace both serial and parallel ports over the next several years. Parallel ports are a more recent invention and are much faster than serial ports. Serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer.Īll computer operating systems in use today support serial ports. The serial port has been an integral part of most computers for more than 20 years.
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