PDP4-340 Experimental Display System

This document describes the General Purpose Experimental Display System, a computer-controlled display system developed by Digital Equipment Corporation.

Key Components:

1. Modified PDP-4C Computer: Features 8192 words of core memory, a perforated tape reader, and a modified high-speed data multiplexer. Modifications include changes to the real-time control (e.g., adding a variable-frequency clock, a "limbo" state for continuous break cycles, disabling clock overflow) and additional address/save registers for the display. It also includes interface control and status circuits for communication with a CDC 1604A digital computer.
2. Modified Type 340 Incremental CRT Display: A standard display with slight modifications and additional equipment. This includes a Type 370 Light Pen and specific power supplies.
3. Special Operator's Console: An interactive table with pushbuttons, lights, a keyboard, control knobs, and a trackball. Associated electronic circuits process these inputs.
4. Interface Equipment: Connects the system to a CDC 1604A computer, allowing it to function as input/output for the 1604A, though it can also operate independently.

Primary Function:

The system presents information (dots, lines, curves, characters) stored in the PDP-4's memory on a CRT. The operator can generate new data or modify the display file and system operations by using the console's interactive elements (buttons, keys, knobs, trackball, light pen).

Programming:

The 340 Display interprets 18-bit input words in five modes (control, point, vector, vector continue, increment), with operational differences from the standard 340, such as modified skip/escape bit handling and edge violation behavior. The manual details various Input/Output Transfer (IOT) instructions for display control, data transfer, and reading system states (e.g., RESUME, DRA, DJP, DJS). Many standard and special instructions have new mnemonic names. Knob and trackball counters store physical movements, allowing for interaction even during overflow conditions.

Maintenance:

The manual provides guidance on diagnosing and correcting mechanical, power, and logic failures, with an emphasis on preventive maintenance through diagnostic programs and marginal voltage checks. It also describes a 1604A simulation method using special test cables to verify the interface electronic circuits.