VT420

This "VT420 Programmer Reference Manual" provides comprehensive information for developing applications that interact with the Digital Equipment Corporation VT420 text terminal. It is designed for programmers familiar with computer programming concepts and character encoding systems.

The manual covers the following key areas:

1. VT420 Features Overview:

   • Models: Describes North American, Worldwide, and Worldwide with PC TERM models, highlighting differences in character set support and communication ports.
   • Keyboards: Details ANSI, Short ANSI, and PC keyboard layouts and their respective functionalities.
   • New Features: Emphasizes advanced capabilities like PC TERM mode (for PC application emulation), dual sessions (MSC or SSU protocol), user windows (for simultaneous session viewing), extensive page memory (storing multiple screens of text for faster updates), macro programming (downloadable ANSI text/commands), rectangular area operations (copy, erase, fill, attribute changes), and local copy/paste.
   • Display/Text Features: Covers screen display options (24, 36, or 48 lines), various built-in character sets (ASCII, DEC Supplemental, ISO Latin-1, National Replacement, DEC Special Graphic, DEC Technical, PC character sets), and the ability to define and download soft character sets. It also details visual character attributes (bold, underline, blinking, reverse, invisible) and line attributes (single/double width, single/double height).
   • Operating Modes: Explains the primary modes: VT400 (default, full VT420 capabilities), PC TERM (PC application support, sends scan codes), VT100 (7-bit, restricted for compatibility), and VT52 (Digital's private control functions).

2. Character Encoding:

   • Standards: Aligns with ANSI and ISO standards for 7-bit and 8-bit character encoding.
   • Code Tables: Details 7-bit ASCII and 8-bit code tables, including C0 (7-bit) and C1 (8-bit) control characters and their functions (e.g., cursor movement, flow control).
   • Control Functions: Defines the structure of multi-character control functions: Escape Sequences, Control Sequences (CSI-based with numeric/selective parameters), and Device Control Strings (DCS-based for operations like downloading character sets or user-defined keys).
   • Macros: Explains how to define (DECDMAC) and invoke (DECINVM) macros, which are stored strings of ANSI text and commands.
   • Display Controls Mode: A special mode for debugging, allowing control characters to be displayed as graphic characters.

3. Control Functions Sent to the Host:

   • Describes the specific codes generated by the keyboard's various keypads (main, editing, numeric) and function keys (top-row) in different operating modes (ANSI, Short ANSI, PC) and how 7-bit control characters are sent.

4. Control Functions Received from the Host: This is a core section for programming:

   • Emulation: Details how the VT420 emulates previous VT terminals (VT100, VT200, VT300) and lists control functions ignored in Level 1 (VT100 mode). It also explains how to select operating levels and send C1 controls as 7-bit or 8-bit characters.
   • Character Set Management: Covers designating (SCS sequences) and mapping (locking/single shifts) character sets into the terminal's active GL/GR tables.
   • Page Memory Control: Explains functions for managing the multi-page memory, including setting columns (DECSCPP, DECCOLM), lines (DECSLPP), margins (DECSLRM, DECSTBM), origin mode (DECOM), vertical split screen mode (DECVSSM), and moving between pages (NP, PP, PPA, PPR).
   • Visual and Line Attributes: Describes how to apply visual attributes (SGR) and line attributes (DECSWL, DECDWL, DECDHL) to text.
   • Editing Functions: Provides control functions for inserting/deleting characters, lines, and columns, and erasing text (ED, EL, ECH, DECSCA for selective erase).
   • Rectangular Area Operations: Comprehensive commands for copying, filling, erasing, and changing/reversing attributes within rectangular text areas (DECCRA, DECFRA, DECERA, DECSERA, DECSACE, DECCARA, DECRARA).
   • Cursor Movement and Panning: Details functions for precise cursor positioning (CUP, HVP, CUF, CUB, CUU, CUD) and panning the user window (SU, SD), including cursor coupling modes (DECVCCM, DECPCCM).
   • Keyboard, Printing, and Display Commands: Covers programming the keyboard (e.g., keyboard action mode, autorepeat, cursor key mode, user-defined keys), controlling the printer port, and managing screen display (e.g., local echo, screen mode, scrolling mode, status line configuration).

5. VT420 Reports:

   • Explains how the host can request and receive reports on the terminal's current state, including device attributes (DA), various device status reports (DSR for operating status, cursor position, printer, UDKs, keyboard, macro space, memory checksum, data integrity, multi-session), checksums of rectangular areas, terminal state reports, presentation state reports, and user-preferred supplemental set reports. These reports are crucial for applications to adapt to the terminal's capabilities and current configuration.

6. Resetting and Testing:

   • Describes control functions for resetting the terminal to various states (soft reset, hard reset/RIS, secure reset) and for performing diagnostic tests (screen alignment, confidence tests).

7. Session Management:

   • Details the independent and shared resources when running two sessions and the two primary methods for managing them: Multiple System Communications (MSC) and the proprietary SSU software.

8. PC TERM Mode:

   • A dedicated section explaining how the VT420 operates as a PC emulator, including its unique scan code handling, specific character sets, and how it interacts with PC applications.

In essence, this manual serves as a definitive guide for programmers to understand and precisely control the VT420 terminal's diverse functionalities, from character encoding and display management to keyboard interactions and multi-session operations, enabling tailored application development.