Almost all modems conform to the EIA-232 standard and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE) and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).

The EIA-232 standard supports two types of connectors -- a 25-pin D-type connector (DB-25) and a 9-pin D-type connector (DB-9). The type of serial communications used by PCs requires only 9 pins so either type of connector will work equally well.

Although EIA-232 is still the most common standard for serial communication, the EIA has recently defined successors to EIA-232 called RS-422 and RS-423. The new standards are backward compatible so that RS-232 devices can connect to an RS-422 port.

Introduction to Serial Communication
Good tutorial on RS232 (serial) communications.

RS-232 connectors and cabling
Provides the 25 pin assignments for the RS-232, along with alternative serial connectors (DIN-8, DB-9, and RJ-45). In addition, DTE and DCE cable connections are discussed.

RS-232 Standard Tutorial
Provides a tutorial with signal names and definitions. Also includes links to EIA publications.

RS-232, RS-422 and V.35 interfaces
Provides technical information on the RS-232, RS-422, and V.35 interfaces.