CRT - Cathode Ray Tube
A Cathode Ray Tube (CRT) is an obsolete vacuum tube technology that produced images by firing an electron beam onto a phosphorescent screen. In the mainframe context, CRTs were the core display component of dedicated **3270 terminals**, serving as the primary user interface for interacting with z/OS systems and applications.
Key Characteristics
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- Display Technology: Utilized an electron gun to project a focused beam of electrons onto a phosphor-coated screen, causing it to glow and form characters.
- Monochromatic Output: Most mainframe CRTs displayed text in a single color (e.g., green, amber, or white) on a dark background, optimized for readability of character-based data.
- Character-Based Interface: Designed to display fixed-size characters (alphanumeric and special symbols) within a grid (e.g., 24x80 or 32x80), rather than pixel-based graphics.
- Dedicated Hardware: Integrated into dumb terminals like the IBM 3270, which had built-in logic for screen formatting, field attributes (e.g., protected, unprotect, high-intensity), and communication protocols (e.g., SNA/SDLC).
- Burn-in Susceptibility: Prolonged display of static images could lead to "ghosting" or permanent discoloration of the phosphor, a phenomenon known as screen burn-in.
- Bulky and Heavy: Due to the vacuum tube and associated electronics, CRT monitors were significantly larger and heavier than modern flat-panel displays.
Use Cases
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- Operator Consoles: System operators used CRT-based 3270 terminals to monitor system status, view console messages, and issue commands to the z/OS operating system.
- Application Interaction: End-users interacted with mission-critical applications running under CICS, IMS TM, or custom-built transaction processors, performing data entry, queries, and updates.
- Program Development: Developers utilized TSO/ISPF sessions on CRTs to write, edit, compile, and debug COBOL, PL/I, Assembler, and JCL programs.
- Database Administration: Database administrators used CRTs to manage DB2 and IMS DB databases, executing SQL queries, utilities, and monitoring performance.
Related Concepts
CRTs were integral to the IBM 3270 terminal family, which connected to the mainframe via VTAM (Virtual Telecommunications Access Method) and SNA (Systems Network Architecture) protocols. They provided the visual interface for interactive mainframe subsystems like TSO/ISPF, CICS, and IMS TM, allowing users to interact with applications and data. Modern 3270 emulators on PCs effectively simulate the behavior and display characteristics of these legacy CRT terminals, enabling continued access to mainframe applications.
- Ergonomics: Position the CRT screen at an appropriate distance and angle to reduce eye strain and promote good posture for users, a common concern in the era of widespread CRT use.
- Screen Savers: Implement screen saver utilities or ensure applications blanked the screen after periods of inactivity to prevent permanent burn-in of static images on the phosphor.
- Regular Cleaning: Periodically clean the screen surface to remove dust and smudges, which could degrade image clarity and contribute to eye fatigue.
- Migration Planning: For organizations still relying on physical CRTs (e.g., in legacy control rooms), plan for migration to modern thin clients or PC-based 3270 emulators to leverage current hardware and reduce maintenance overhead.
- Disposal: Adhere to environmental regulations for the proper disposal of CRTs, as they contain lead and other hazardous materials.