Chapter 22. Serial Communications

Despite the increasing popularity of USB, serial communications remain important to many PC users. PCs use serial communications to connect peripherals such as mice, modems, digital cameras, barcode readers, and similar low-speed devices. In one sense, serial communications are straightforward. If you use the right cable and let Windows take care of configuring parameters, everything usually just works. When it doesn’t, however, you need to understand quite a bit about the underlying plumbing to make things right. This chapter describes what you need to know about PC serial communications.

Serial Communications Overview

In serial communications, bits are transferred between devices one after the other in a series, hence 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 ...

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