DEC-89-H1SA VA38 Character Generator Maintenance Feb69

This document is an instruction manual for the VA38 20µs Character Generator, an optional module manufactured by Digital Equipment Corporation (DEC) in 1969 for their Type 338 or 339 Programmed Buffered Displays.

Key features and functions include:

• Purpose: To generate and display ASCII (American Standard Code for Information Interchange) characters on the connected display.
• Performance: Capable of generating characters at 20 microseconds per character, allowing for 1600 flicker-free ASCII characters.
• Architecture: Comprises control logic and a read-only memory (ROM) implemented using DEC FLIP-CHIP modules.
• Character Generation: ASCII character codes are combined with an internal 4-bit word to form a 10-bit address for the ROM. The ROM outputs 6-bit character words, which are decoded by the VA38's CH Register to control display elements such as character intensity, number of drawing moves, and direction of movement.
• Special Characters: It processes specific ASCII codes as "special" characters (Carriage Return, Line Feed, Escape) which trigger distinct display functions like moving the cursor to the left edge, advancing a line, or exiting character mode, respectively.
• Intensity Control: Utilizes a specialized Type 4698 Intensity Amplifier module, which replaces an older model (4688) to provide faster intensify times (100ns) and allows individual adjustment of eight programmable intensity levels for improved display flexibility and responsiveness.

Installation and Maintenance:

• Installation is typically performed by a DEC Field Engineer, involving mechanical mounting, electrical cabling, and specific modifications to the display system, including the replacement of the intensity amplifier and adjustments to display delays.
• Programming involves a mode instruction (Mode 7, command 1171) to send character data (two characters per PDP-8 memory word) to the generator.
• Maintenance uses diagnostic programs (e.g., MAINDEC-08-D8MA) to check electrical parameters, signal pulses, memory operation, and to perform adjustments, particularly for the 4698 Intensity Amplifier.