ZyXEL Elite 2864 User Guide - Page 28

RS-232C or EIA-232D/E RS-232C is the Recommended Standard (RS) of the Electronic Industries Association (EIA), defining the serial communication interface between a DTE and a DCE. The 232 is basically a serial number for the defined standard. Sometimes it is necessary to redefine a standard, or to revise it. The most commonly used revision of the RS-232 standard is the "C" revision. For the "D" revision, the prefix was changed to EIA. Except for a few added, but not commonly used signals, there is no practical difference between the "C" and "D" revisions. There is now a new revision with the "E" suffix. The RS-232C standard is equivalent to the ITU-T V.24 and V.28 standard. Also refer to the Modem Standards and Speeds section below. Serial Port A serial port is the serial data connector together with its internal circuit on the DTE or DCE with electrical and mechanical characteristics according to RS-232C. Since some signals are going from the DTE port to DCE port, and some signals are going in the other direction, the signal pin is a transmitter on one port and a receiver on the other. The DTE serial port is different from the DCE serial port in terms of signals on the connector pins. There are also mechanical differences in terms of male (with pins) or female (with holes) connectors. Serial RS-232C Cable A serial RS-232C cable is used to connect a DTE port to a DCE port. Do not use a null-modem cable (which may be used to connect two DTEs directly with each other through their serial ports). A normal RS-232C connector has 25 pins and a normal RS-232C cable has 25 wires. Many signals in the RS-232C are not used in common applications and a 9-wire RS-232C cable is sufficient in most applications. The PC-AT's serial port has only 9 connector pins, eliminating the unnecessary pins. For high-speed DTE-DCE comunication, use a low-capacitance cable, as short as possible. Synchronous and Asynchronous Communication There are two kinds of serial data communication. One is called synchronous and the other is called asynchronous. In synchronous communication, data is transmitted and received bit by bit and is timed by an accompanying clock signal. In asynchronous communication, data is sent character by character (or octet by octet), and the idle time between characters is variable. No clock signal is sent; character timing is recovered from the data itself. 2-2