PC Hardware in a Nutshell, 3rd Edition

Asynchronous Frame Types

Many different asynchronous frame types can be created by using different combinations of data word length, parity type, and number of stop bits. Frame types are abbreviated using the number of data bits, the parity type, and the number of stop bits. For example, a frame that uses seven data bits, even parity, and one stop bit is abbreviated 7E1, and one that uses eight data bits, no parity, and one stop bit is abbreviated 8N1. These, incidentally, are the only two frame types commonly used in PC serial communications. The 7E1 frame type is still sometimes used for direct and dial-up serial connections to mainframes and for connections to old serial printers. The 8N1 frame type is used for nearly everything else.

Tip

Because most asynchronous frame types use eight data bits (or seven data bits plus a parity bit) framed by one start bit and one stop bit, a serial link typically requires 10 bits to transfer each eight-bit byte, or 20% overhead. That means you can calculate the actual number of bytes per second transferred on an asynchronous serial connection as simply 1/10 of the bits/s rate. For example, a serial terminal connected to a Linux system at 38,400 bits/s transfers 3,840 bytes/s. This calculation does not hold for modern modems, which initially link asynchronously, but switch to synchronous mode once the connection is established. In synchronous mode, each byte requires only eight bits, plus the minor overhead required for framing and error correction ...