Voltage sent over the pins can be in one of two states, On or Off. Not Used: Receive Current Loop Return (-).Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected.Data Terminal Ready - Computer tells the modem that it is ready to talk.Not Used: Receive Current Loop Data (+).Not Used: Transmit Current Loop Data (-).Not Used: Transmit Current Loop Return (+).Received Line Signal Detector - Determines if the modem is connected to a working phone line.Data Set Ready - Modem tells the computer that it is ready to talk.Clear To Send - Modem tells the computer that it can send information.Request To Send - Computer asks the modem if it can send information. Receive Data - Computer receives information sent from the modem.Transmit Data - Computer sends information to the modem.While most standard serial ports have a maximum transfer rate of 115 Kbps (kilobits per second), high speed serial ports, such as Enhanced Serial Port (ESP) and Super Enhanced Serial Port (Super ESP), can reach data transfer rates of 460 Kbps. This buffer allows the chip to cache data coming in from the system bus while it is processing data going out to the serial port. In order to function faster, most UART chips have a built-in buffer of anywhere from 16 to 64 kilobytes. The UART chip takes the parallel output of the computer's system bus and transforms it into serial form for transmission through the serial port. Serial ports rely on a special controller chip, the Universal Asynchronous Receiver/Transmitter (UART), to function properly. Using different pins allows for full-duplex communication, in which information can travel in both directions at once. Serial devices use different pins to receive and transmit data - using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time. Bi-directional communication allows each device to receive data as well as transmit it. Serial ports, also called communication (COM) ports, are bi-directional. 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, which is a single bit with a value of 0.
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