Serial Communication Overview 
Outlines the technology behind the PC modem, the associated CCITT standards and higher performance solutions such as ISDN, and xDSL. Includes graphics and diagram. This page is from "The PC Technology Guide."
Introduction to Serial Communication
Good tutorial on RS232 (serial) communications.
PC communications over modems and ISDN
This article, from August 1996, describes serial ports, introduces the most general and least expensive ISDN equipment, and provides an outline of the steps to exploit this new service.
Serial Port Central
Information, links, tools, and code for accessing serial ports, including RS-232/RS-423 and RS-485/RS-422. Includes links to tools for serial-port access under Windows and information on specifications, wireless communications, and interface chips.
Serial Port Connection
Describes the various serial port devices, with DB9 and DB25 information and illustrations and steps on how to configure a direct cable connection in Windows 95 / Windows 98.
Troubleshooting Peripheral I/O Ports
Discusses troubleshooting of peripheral I/O ports on the PC including serial and parallel I/O port connections. This page is from "The PC Guide."