ASCII 17 — DC1

Device Control 1 (DC1), widely known as XON, is part of the XON/XOFF software flow control mechanism in serial communication. When a receiving device is ready to accept more data, it sends DC1 (XON) to tell the sender to resume transmission. Generated by Ctrl+Q, DC1 remains relevant in terminal emulators — accidentally pressing Ctrl+S (XOFF/DC3) can freeze terminal output, and Ctrl+Q (XON) resumes it. This behavior still catches developers off guard when their terminal appears to hang unexpectedly.

Control characters were defined in the original 1963 ASCII standard to manage telecommunications equipment and terminal devices. Unlike printable characters representing visible symbols, control codes perform actions: initiating transmissions, acknowledging received data, triggering device alerts, and structuring information hierarchically. Of ASCII's 128 code points, 33 are designated as control characters (codes 0–31 plus 127), reflecting the standard's deep roots in telegraphy and serial communication systems. While most control codes have fallen out of daily use, several remain essential to modern computing workflows.

In the ASCII encoding table, Device Control 1 (XON) is assigned code point 17 in decimal (0x11 hexadecimal, 021 octal, 00010001 binary). The 7-bit ASCII standard, first published in 1963 by the American Standards Association, defines exactly 128 characters that remain the foundation of text encoding systems worldwide. UTF-8, the dominant encoding on the modern web, is fully backward compatible with ASCII — every ASCII character is encoded as the identical single byte in UTF-8, guaranteeing that Device Control 1 (XON) works reliably across all operating systems, programming languages, and internet protocols.