The CDC 6600 system console was a vector drawing system with a single font that also provided a way of drawing simple graphics (Wikipedia).

In simple terms how does this vector drawing system work?

Further, how did the dual screens operate and how were they used? Particularly interesting may be the simple graphics functionality.

An additional but lesser question is what font was used?


In simple terms how does this vector drawing system work?

Like essentially like an oscilloscope. A beam is used to draw on the screen, controlled by two voltages for X/Y. Not much different as a TV, except with TV the movement is fixed as lines left to right and top down.

In very simple terms: (*1)

A CRT is a huge tube with a heated electrode supplying a constant stream of electrons (*2) focused to hit another electrode which happens to be the screen. When an electron hits, its energy gets turned into photons aka light. The beam can be modulated (on/off) and directed by applying voltage to a set of coils. One deflecting in horizontal direction. Negative for up, positive for down. The other coil does the same vertical. Positive to the right, negative to the left. Changing the voltage allows to draw any arbitrary set of lines or vectors as they are called.

For a text vector display one would sort all vectors for the letters to be drawn in sequence of characters and vectors within a character.

For musings about how such displays can be made, take a looks at this question:

What are principles of vector CRT display?

In case of the 6600 console all drawing was done from CPU memory by a dedicated I/O processor, which reads a stream of XY coordinate to be drawn and sends them to a DA converter within the console to to be turned into voltages to the coils to deflect the beam.

[In some way this is a bit like used by 'cheap video' and later Sinclair's ZX80/81 computers. The (A) CPU (Peripheral Processor) feeds a data structure in RAM to a rather primitive video hardware only doing the signal conversion.]

Further, how did the dual screens operate and how were they used?

Simply as two screens. One for interactive input one for a log. Or one as system console one as user output (graphics). There was no hardware preference for either screen. Much like today one could use two windows on a PC - or two screens.

Of course users got creative - like with every new toy. There was a chess program that did draw board and pieces on one screen while having a textual list of all moves on the other. And then there was eventually the first implementation of the 'EYES' program, well known from X-Window. Here each screen displayed an eye, both looking around (synchronized), blinked and so on. Of course not following a mouse pointer, but it could be controlled.

Particularly interesting may be the simple graphics functionality.

It wasn't simple (*3) in any way, as next to arbitrary vector graphic could be displayed. As a rough estimation a peripheral processor can output one value per micro second, two are needed for a vector, or 500,000 vectors per second. Lets say we want to keep the refresh rate at 50 Hz (*4), then a graphic displayed can be made up from up to 10,000 lines - more with lower refresh rates (*5).

An additional but lesser question is what font was used?

A readable? Not sure what you expect as an answer here. At this time in history there were no artistic choices. It was all about getting some text out - which was already seen as a major achievement. What can be said is that characters were designed to use as little vectors (strokes, visible or invisible) as possible, as the total number of vectors displayed defined the refresh rate.

Characters were thus rather angled, but seamed, thanks to their vector nature way more clean and readable than any contemporary raster display:

enter image description here

(Picture taken a cray-cyber.org blog entry)

The shown CC545 console is a later model. Workings are still the same, including the ability to produce two pictures, but only a single CRT is used. A switch, labeled left/right, seen in the lower left, beside the keyboard, is provided to select between either output , here set to display the right screen.

*1 - REALLY simple. It's as simple as I can. So please, no arguing about finer details, correct names, etc.

*2 - In a tube the electrode emitting the electron ray is called cathode - hence the name Cathode Ray Tube or short CRT.

*3 - I guess the one adding that wording would qualify all vector display as 'simple' in hindsight.

*4 - Refresh rate on vector displays is not a constant but depends on the number of vectors displayed.

*5 - 50 Hz is a rather high refresh rate for vector displays, as these displays do usually feature a longer persistence than later raster screens. As low as 20 Hz can be still fine.

  • 1
    Morgen frueh editlib laufen lassen? JA
    – OmarL
    Nov 28 '20 at 18:26
  • 1
    @SingleMalt No, it does draw lines. It does not have pixels. Output is essentially a series of 'DRAW-TO' commands, either with the cannon enabled, thus producing a line, or cannon disabled, resulting in a repositioning. So for all practical purpose the the pixel density between two points was indeinite. What was quantisized is the number of addressablepositions for start or end of a line/positioning,which was (IIRC) 12 bit, or 4096x4096 possiblelocations.
    – Raffzahn
    Nov 28 '20 at 20:55
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    @OmarL Ja :)) The stickers are still preserved from the original installation, before the machine was taken out of service. We believe that these machines tell a story, and 'restoring' to a clean, better than new state is essentially destroying its history.
    – Raffzahn
    Nov 28 '20 at 20:57
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    More w.r.t. lack of pixels: On a monochrome display the entire surface of the CRT is a uniform phosphor coating. A vector display can draw anywhere on it, and in continuous lines. A color CRT had either stripes or dots of different color phosphors (this is independent of the "pixel' nature of the raster scan displays!) and that led to graininess and lack of resolution. There is no display as clear as a high resolution monochrome display and I'd gladly go back to one in an instant - if it accepted modern interfaces - and leave behind the "color coded syntax" of modern IDEs.
    – davidbak
    Nov 28 '20 at 21:38
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    @SingleMalt - I don't think "momentum" is the right word - I'm not sure of the right word actually - but the beam direction is affected by inductance and capacitance in the circuitry changing the electric and/or magnetic fields controlling the beam, and that causes corners to get rounded off, or overshot, depending.
    – davidbak
    Nov 28 '20 at 22:02

Specifically a sub-answer with respect to fonts: Don't know about the CDC 6600 but in the vector graphics world the Hershey fonts ruled - at least until the very late 70's, early 80's. And for plotters - perhaps even later, as they were provided by all the plotting libraries you could get.

Here are some examples of what they looked like, and here is Hershey's original technical report which has more information than you ever wanted to know about the constraints under which they were designed, and how they were designed (including mathematical formulas!).

(Almost forgot: Extremely important in establishing their ubiquity was that they were free! Developed at great expense and with great care by the US military they were available for all to use.)

Here's "Simplex Roman":

Hershey's "Simplex Roman" font

And here's "Complex Script" which was for some reason very widely used, even though even back then - not to mention now to the modern eye - it looked like ... well, hmm, amateur? (If not worse.)

![enter image description here

(Answer truthfully, you prefer Comic Sans to that, right?)

  • Interesting to know about this. The CDC 6600 was released in 1964 so slightly predates these fonts (Wikipedia says circa 1967). But it could be that they shared a common ancestor design. Nov 28 '20 at 22:15
  • @SingleMalt it wasn't terribly difficult to design your own vector font. I did one around 1974 or so. P.S. that Simplex Script looks a lot like MS Script for Windows, I wonder if that's where they obtained it? Dec 4 '20 at 1:43
  • @MarkRansom That vector fonts were not difficult to design your own is interesting. Further, the Hershey Wikipedia page says that vector fonts are easily scaled and rotated in two or three dimensions. “Some glyphs were developed in four different versions, dubbed Simplex, Duplex, Complex and Triplex, which used different numbers of strokes to compose their contours” [again Wikipedia], suggesting further flexibility. The two example fonts above are simplex, so common terminology was in use. Dec 6 '20 at 9:45

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