PC Hardware in a Nutshell, 3rd Edition

Asynchronous Serial Framing

In asynchronous serial systems, data is broken into five- to eight-bit words, with seven- and eight-bit words by far most common. Seven-bit words are used for text-mode transfers because seven bits accommodate the entire low-order ASCII character set. Eight-bit words are used for binary-mode transfers (and may be used for text-mode transfers) because using eight bits allows the entire data byte to be transferred as a single unit.

When assembling the data to be transmitted, the serial interface adds control bits to the data block to create a frame. The frame begins with the start bit, a special double-length pulse that signals the receiving interface that transmitted data is about to begin arriving. Immediately following the start bit, the frame contains the data word, with the least-significant bit first. Immediately following the final data bit, the frame may contain a parity bit. The frame ends with one or more required stop bits, which signal the receiving interface that the frame is complete. When assembled, the start bit, data bits, parity bit, and stop bit together make up one frame.