A protocol for controlling data flow between computers or other devices on an asynchronous serial connection. The “X” means “transmitter” and the Xon and Xoff are signals to turn a transmitter on or off. Practically speaking, the signal for Xon is the same bit configuration as the ASCII Ctrl-Q keyboard combination, whereas that for Xoff is the same bit configuration as the ASCII Ctrl-S character. As a case in point, it is quite common for a computer to send data to a printer faster than a printer can execute the job. The printer contains a buffer in which data are stored until the printer can meet the computer’s demand. If the buffer becomes full before the printer can meet the demand, a small microprocessor in the printer sends an Xoff signal to the computer to have it stop sending data. When enough data are printed and buffer storage is available, the printer sends an Xon signal to the computer, signaling that data sending can be resumed.