ASCII Character ␑Device Control One (XON)
The device control codes (DC1 to DC4) are reserved for device control, with the interpretation dependent upon the device to which they were connected. DC1 and DC2 were intended primarily to indicate activating a device while DC3 and DC4 were intended primarily to indicate pausing or turning off a device. DC1 and DC3 (known also as XON and XOFF respectively in this usage) originated as the "start and stop remote paper-tape-reader" functions in ASCII Telex networks. This teleprinter usage became the de facto standard for software flow control.