Technology Reference Guide

	Technology Reference Guide 6. Terminal Emulation/Host Connectivity E. Asynchronous Communications

What is it?
Asynchronous communication refers to the terminal emulation scheme used by almost all non-IBM computers. It is called asynchronous because, unlike IBMs terminal protocols, it does not use a clock signal to synchronize the data transmission. Each transmitted character contains start and stop bits that convey to the receiver the meaning of the transmitted data stream. Other typical names for this type of terminal include RS-232 terminals, or serial terminals. These types of terminals use a standard serial port connection, known as a DB25 connector, or the smaller DB9 connector, and a common serial cable.

What does it do?
Digital Equipment systems of all sizes, as well as those of other manufacturers use this type of terminal interface. Since most personal computers come equipped with a standard serial port, this provides the physical connection required. However, it is still necessary to have a terminal emulation program to provide the communications protocols required to communicate with the host system. These emulation programs must insure compatibility with the keyboard layout expected by the host. Common emulation schemes are based Digital Equipment terminals such as the VT52, VT100, and VT220 keyboard layouts.

How is it used?
Asynchronous terminal emulation is used in a variety of situations. First, it is used to provide terminal connectivity to compatible host systems with the use of a terminal emulation program. It is also widely used to provide remote connectivity through common modems, in both PC LAN and host environments using terminal emulation or remote control software. On-line services generally use asynchronous terminal emulation as the standard user interface.

Where do I get more information?
Contact your local CBV Office.