splay terminals are widely used in inquiry-response appli- cations as "electronic blackboards" to provide rapid access to data stored in computer systems. These terminals are preferred due to their ability to selectively alter the displayed data. To provide data processing services, an intelligent terminal should provide operating system, arithmetic and logic functions, as well as handle local files. It should also be able to run application programs 52 TELECOMMUNICATIONS Fig. 4-1. CRT terminal from Hewlett-Packard. (Courtesy Hewlett-Packard.) written in a high-level language, and to communicate with the host processor over a telephone line. To fulfill these requirements, most intelligent terminals employ hardware modules consisting of a control unit, an alphanumberic keyboard, and a display screen. 4.1.1 Control Unit The control unit (Fig. 4-2) directs and services the needs of the ter- minal. The control unit usually is integrated into the terminal, but in some cases it's a separate unit. Generally, separation is used when the terminals are configured in a cluster, where two or more termi- TERMINAL TECHNOLOGY 53 Basic functions Communication line Buffering Line discipline (polling) Screen regeneration Modem Error control Advanced functions Editing Data compaction Input validation Control unit Man-machine dialogue Graphic manipulation Terminals Fig. 4-2. Intelligent terminal control unit functions. nals are handled by one controller. The control unit usually contains a microcomputer supported by a random access memory (RAM). The RAM is used to handle application programs, data files, system utilities and, in some cases, the operating system. 4.1.2 Keyboard Structure The keyboard is generally laid out in typewriter-style, although some terminals with advanced text processing capabilities may furnish a slightly modified version. Associated with the keyboard can be a sep- arate keypad which allows entry of numeric data. All keyboards pro- vide separate keys for positioning the screen cursor, editing local data, and