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Episode 19 Notes - Early PLATO

Dec 19, 2019

PLATO(Programmed Logic for Automatic Teaching Operations) was an ambitious project started in 1960 with the goal of revolutionizing learning. And despite massive technical advances made by the project it remains relatively obscure today. It started out small, with just one terminal connected to the University of Illinois' ILLIAC computer, but over time PLATO evolved into a robust computerized teaching platform. Over the projects lifespan there were four major interactions: PLATO I, II, III, and IV. The first two versions were essentially prototypes. PLATO I was the single terminal I described above, PLATO II was similar to the first version but allowed up to two terminals connected to ILLIAC to be used simultaneously. PLATO III was the first version to see actual use in classrooms, with some of the first college courses being offered on the platform in 1965.

What's so remarkable about PLATO is that fact that it was able to offer a markedly modern experience decades before any other computers could. The later PLATO IV would introduce things like plasma displays, touch screen interfaces, and networked applications and game. As impressive as those advances are, I think PLATO III is a shocking achievement in it's own right that should not be overlooked.

The core of PLATO III is a mix of old and new technology. Terminals, which connected up to a CDC 1604 mainframe, were actually modified TVs with custom built keysets. Signal flow into the mainframe was relatively simple, each keyboard fed into a multiplexer/switcher then into the computer. The output side of things is a lot more convoluted, but makes a good example of what can be done with limited technology. Two sources were combined to make the final image displayed at any given terminal: a computer controlled slide projector and a storage tube. The slide projector was used as a bit of a work around to avoid loading too much data into limited computer memory, static images could just be turned into slides and loaded up. Some clever programming made it easy to select which slide to display at any given moment. The actual image from the slide was picked up using a scanner. That image was mixed with the image on a storage tube, essentially a CRT tube with that could persist a bitmap image for a short period of time. These tubes were used for more dynamic content that actually needed to be rendered by the computer, and there was a tube dedicated to each terminal. Once mixed together the signal was broadcast over a CCTV setup to it's designated terminal. It made for a robust way to combine graphics and text using the limited computers of the time.

PLATO III was also remarkable for the fact that it was an early time sharing system. Using an operating system called CATORES, written in house, up to 20 terminals could be used simultaneously. This was developed concurrently to other early time-sharing systems like MIT's CTSS, but what I find interesting is that while MIT had an entire project dedicated to time-sharing for PLATO it was simply a step in the road. That seems to be true of a lot of things developed for PLATO, huge advances were made as a matter of convenience.

If you want to learn more about the development of PLATO, here is my episode on the topic:

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