Imlac PDS-1 - Refreshed Vector Display

Refreshed Vector Display

The monitor was a 14-inch monochrome vector display, continually refreshed from local memory. Its normal resolution was 1024 by 1024 addressable points, and 2K x 2K in small-font scaling mode. The CRT electron beam moved freely in X and Y position and angle under program control to draw individual sloped lines and letter forms, much like the pen-on-paper motions of a pen plotter. The beam skipped blank areas of the screen. Things could be drawn in arbitrary order.

Vector displays are a now-obsolete alternative to raster scan displays, where the image is a grid of pixel spots and the CRT beam repeatedly sweeps the entire screen in a fixed horizontal pattern like a television, regardless of which dots are turned on. Bitmap raster graphics requires much more memory than vector graphics. XGA-level 1024x768 black/white resolution requires 96 kilobytes of video refresh memory, 12 times more than a basic PDS-1. In 1970, that much core memory cost about $8000. (It now costs only 0.05 cents of shared DRAM.)

Vector displays were good for showing data charts, modifying line drawings and CAD diagrams, tumbling 3-D wire-frame shapes, editing text, laying out printed pages, and playing simple games. But they did not handle colors, images, filled-in areas, black-on-white screens, or WYSIWYG fidelity to the fonts of professionally printed text.

The PDS-1 screen was repeatedly refreshed or redrawn 40 times per second to avoid visible flickering. But irregular beam motion was slower than the steady motions on raster displays. The beam deflections were driven by magnetic coils, and those coils fought against rapid changes to their current. The screen flickered when filled with more than 800 inches of lines or more than 1200 characters, because the beam then needed more than 1/40th of a second to retrace everything.

The competing lower cost Tektronix 4010 graphics terminal used an alternate storage tube CRT technology which required no continual refresh and hence no local computer memory at all. The glowing image was remembered by the CRT phosphor itself. But like an Etch a Sketch, the accumulated image could be modified or moved only by flash-erasing the entire screen and then slowing redrawing everything with data resent from some large computer. This was much less interactive than the PDS-1 and could not show animations.

On other displays of this era, text fonts were hardwired and could not be changed. For example, the operator consoles of the CDC 6600 formed each letter all at once by sending the Charactron CRT electron beam through a metallic stencil mask with an A-shaped hole, or through a B-shaped hole, etc.

But on the PDS-1, all letter shapes, sizes, and spacing were entirely controlled in software. Each desired form of the letter E had its own display subroutine which executed a sequence of short vector strokes for that letter. Each occurrence of a letter on the screen was a display processor call to that letter's subroutine. This scheme handled arbitrary fonts, extended character sets, and even cursive right-to-left languages like Arabic. The smaller, fastest-drawing fonts were ugly, with diamond-shaped approximations of rounded loops. The display subroutine scheme also handled electronic design symbols.

The PDS-1 monitor face was rectangular and was available in portrait or landscape orientation. The 1K x 1K grid of points was stretched 33% in the longer direction to allow text and graphics to fill the screen. All graphics programs then had to account for the non-square pixels. If the system was to be used mainly for graphics, the monitor could be installed with an unstretched grid leaving ends of the screen permanently unused.