XOFF

XOFF is a control character used in software flow control for serial communications to request that the transmitting device pause data transmission. It is typically paired with XON, which signals that transmission may resume. XOFF is part of a simple in-band signaling scheme that allows devices to manage data flow without a separate control channel.

In the ASCII character set, XOFF is DC3 (decimal 19, hexadecimal 13). XON is DC1 (decimal 17, hexadecimal 11). When a receiver’s buffer fills, it can send XOFF to the sender to halt transmission; the sender continues only after receiving XON. This mechanism is common in asynchronous serial links, such as RS-232 or RS-485, especially when hardware flow control lines (RTS/CTS) are unavailable or not used.

Operation and limitations: software flow control relies on the correct interpretation of the XON/XOFF characters by both ends of the connection. Data bytes that coincidentally resemble XON or XOFF can cause unintended pauses unless the connection is configured for transparency or escaping. Some implementations allow disabling XON/XOFF in favor of hardware flow control or higher-level flow management. Noise on the line can also cause false triggers, leading to data loss or unnecessary pauses.

Modern context: XON/XOFF remains available for legacy equipment, terminal emulators, and certain modem configurations, but many contemporary systems prefer hardware flow control or protocol-level flow control. Its use tends to be limited to scenarios where simple, in-band signaling is sufficient and transparent data transmission is critical.