PC Hardware in a Nutshell, 3rd Edition

Serial Communications Overview

In serial communications, bits are transferred between devices one after the other in a series, whence the name. To communicate an eight-bit byte, the transmitting serial device breaks that byte into its component bits and then places those bits sequentially onto the serial communications interface. The receiving interface accepts the incoming bits, stores them temporarily in a buffer until all bits have been received, reassembles the bits into the original byte, and then delivers that byte to the receiving device.

Because any bit is indistinguishable from any other bit, serial interfaces must use some means to keep things synchronized between the transmitting and receiving interfaces. Otherwise, for example, if transmitted bit #4 were lost due to line noise or some other communication problem, the receiving interface would assume when it received bit #5 that that bit was bit #4, resulting in completely scrambled data. Two methods may be used to effect this synchronization:

Synchronous serial communication

Synchronous serial communication is so called because the transmitting and receiving interfaces are synchronized to a common clock reference. Because both interfaces always “know what time it is,” they are always in step, and always know which bit is on the wire at any particular time. Synchronous serial communication methods are common in mainframe and minicomputer environments, but are little used in PC communications. Synchronous serial communications ...