PDP-X Technical Memorandum #15

This document, PDP-X Technical Memorandum #15 (July 31, 1967), authored by Henry Burkhardt, describes the "Incremental Plotter Control" system.

The system is designed to drive Digital Incremental Recorders (plotters) manufactured by California Computer Products (e.g., models 563 and 565). These plotters operate using bidirectional rotary step motors for X-axis (drum movement) and Y-axis (pen carriage movement), allowing a pen to draw on graph paper. Key capabilities include:

• Motion Control: Incremental movement in any of eight directions.
• Pen Control: Ability to raise or lower the pen from the paper surface.
• Recording Modes: Supports recording discrete points, continuous curves, and symbols.

Programming the plotter involves:

• Interacting with a Status Register using IOC or IOX instructions. This register contains bits like REQ (request), BUSY, LOW, and ENABLE.
• Sending data bytes via IOW commands, where the lower four bits of the data byte determine the plotter's function.
• Specific data values (0-7) correspond to the eight directions of motion, while others (e.g., 10₈ for pen up, 11₈ for pen down) control the pen's state.

Additionally, the system features operator controls that permit manual single-step or continuous operation of the drum and carriage, as well as manual control of the pen solenoid.