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Software flow control is a method of flow control used in computer data links, especially RS-232 serial. It uses special codes, transmitted in-band, over the primary communications channel. These codes are generally called XOFF and XON (from "transmit off" and "transmit on", respectively). Thus, "software flow control" is sometimes called "XON ...
The XON and XOFF characters are sent by the receiver to the sender to control when the sender will send data, that is, these characters go in the opposite direction to the data being sent. The system starts in the sending allowed state. When the receiver's buffers approach capacity, the receiver sends the XOFF character to tell the sender to ...
Paper tape was a very popular medium for long-term program storage until the 1980s, less costly and in some ways less fragile than magnetic tape. In particular, the Teletype Model 33 machine assignments for codes 17 (control-Q, DC1, also known as XON), 19 (control-S, DC3, also known as XOFF), and 127 became de facto standards. The Model 33 was ...
Universal asynchronous receiver-transmitter. A universal asynchronous receiver-transmitter (UART / ˈjuːɑːrt /) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and ...
One change was to escape a small set of control characters: DLE, XON, XOFF and SYN. These were escaped by inserting a DLE in front of them, and then modifying the character by XORing it with 64. In theory, this meant the packet might be as long as 264 bytes if it originally consisted entirely of characters that required escaping.
Digital Equipment Corporation invented a convention which used 19 (the device control 3 character , also known as control-S, or XOFF) to "S"top transmission, and 17 (the device control 1 character , a.k.a. control-Q, or XON) to start transmission. It has become so widely used that most don't realize it is not part of official ASCII.
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If either of these two octets appears in the transmitted data, an escape octet is sent, followed by the original data octet with bit 5 inverted. For example, the byte 0x7E would be transmitted as 0x7D 0x5E ("10111110 01111010"). Other reserved octet values (such as XON or XOFF) can be escaped in the same way if necessary.