ASCII 19 — DC3

Device Control 3 (DC3), widely known as XOFF, tells the sender to pause data transmission in the XON/XOFF flow control scheme. When a receiving device's buffer approaches capacity, it sends DC3 to prevent overflow. Generated by Ctrl+S, this character inadvertently freezes terminal sessions when users press Ctrl+S intending to trigger the save shortcut — a common frustration among developers working in command-line environments, resolved by pressing Ctrl+Q (XON/DC1) to resume the paused output stream.

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 3 (XOFF) is assigned code point 19 in decimal (0x13 hexadecimal, 023 octal, 00010011 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 3 (XOFF) works reliably across all operating systems, programming languages, and internet protocols.