Applications
Software flow control is used extensively by low-speed devices, especially older printers and dumb terminals, to indicate they are temporarily unable to accept more data. Typically, this is due to a combination of limited output rate and any buffers being full. Some terminal control packages, such as termcap, employ "padding" to allow such equipment sufficient time to perform the requested actions without the need to assert XOFF.
XOFF/XON are still sometimes used manually by computer operators, to pause and restart output which otherwise would scroll off the display too quickly.
Terminal emulator software generally implements XOFF/XON support as a basic function. This generally includes the system console on modern Unix and Linux machines, as well as GUI emulators such as xterm and the Win32 console.
Robust XON is a technique to restart communication, just in case it was stopped by an accidentally received XOFF. The receiving unit sends periodic XON characters when it can receive data, and the line is idle. One common use is by serial printers (like HP LaserJet II) to indicate they are online and ready to receive data. The XON is sent every 1 to 30 seconds depending on the printers' firmware design.
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