stopbits

Stop bits are a component of the framing used in asynchronous serial communications to denote the end of a character and provide a gap before the next character is sent. They consist of one or more bit times during which the line remains in the mark state (logical high). The stop bit count, together with data bits and parity, is negotiated with the baud rate to define a complete character format.

Common configurations use 1 stop bit, while some hardware and older equipment support 1.5 or 2 stop bits. The number of stop bits affects the time required to transmit a character at a fixed baud rate; increasing the stop-bit count reduces the effective data rate because more time is spent sending the stop period. Higher stop-bit counts can improve tolerance to noise and timing mismatches on long cables or at lower speeds, but at the cost of throughput.

Interoperability requires both ends of a connection to agree on the same framing settings. The format is typically expressed together with data bits and parity, for example 8N1 (8 data bits, no parity, 1 stop bit) or 7E2 (7 data bits, even parity, 2 stop bits). Although 1.5 stop bits exist in some legacy or specialized hardware, they are not universally supported and are uncommon in modern systems.

Configuring stop bits is part of UART settings used by various serial interfaces, including RS-232 and RS-485 in asynchronous mode, and by USB-to-serial adapters. Incorrect stop-bit configuration results in framing errors or garbled data, highlighting the need for matching settings on both transmitter and receiver. See also UART, asynchronous serial, baud rate, parity, and data bits.