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I'm reading a book called The Friendly Orange Glow, about the PLATO multiuser computer system developed by the University of Illinois in the late sixties, which is fascinating at several levels as a microcosm of an alternate universe of computing. The particular aspect I'm looking at here is the source of the orange glow, the plasma screen, a key enabling technology for the system.

PLATO used a 512x512 monochrome bitmap display, which apparently cost $2500. (I'm not clear whether that's just for the plasma screen itself or for the entire terminal, which contained additional components such as a 16x16 touchscreen input.) That's a quarter million bits (32 kilobytes) of data. On a CRT or LCD, it would need to be backed by a quarter million bits of video memory. According to https://en.wikipedia.org/wiki/Magnetic-core_memory in the early days of magnetic core, that would've cost a quarter million dollars; even by the early seventies, it would still have been $2500 of video memory, doubling the total cost of the display.

But apparently a plasma screen has the remarkable property that it does not need refresh, i.e. it doubles as its own video memory. And for a few years, that was a key advantage.

I'm trying to reason about what it would be like to use such a system. For example, scrolling would not be a primitive operation. You would always have to redraw the display based on internal data (whose format would be optimized for compactness, not speed of redraw), so it would be natural to design around 'page at a time' instead of continuous scrolling.

I'm also trying to figure out what would be the implications of the display persisting without refresh. It occurs to me that might depend on timescale; just because the display doesn't need refreshing every 1/60 of a second, doesn't necessarily mean it persists forever.

Just how long was the image on a 1970-era plasma screen stable for?

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    PLATO applications separate the screen into regions and then update only the affected region based on user input. You can see for yourself using the PLATO instance at irata.online
    – Brian H
    Feb 21 at 22:24
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    If all you're trying to do is get the experience of using a screen that serves as its own video memory and doesn't need a refresh, just use a Kindle or anything else with an e-ink display. As for the plasma experience, I have a perfectly functional Compaq lunchbox, haven't fired it up in a while but IIRC the fade is somewhere in the 50-250ms ballpark? Something like that. It's in my storage unit. But I'd be happy to take a video of it for you if I have time to go out there this week. But also there were plasma TVs as recently as 2014; they were the top tech for a couple years before OLED.
    – Jason C
    Feb 22 at 17:29
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    Or if you find any old laptops or monitors where you have to have your head in a sweet spot to see it, and if you moved like a half an inch everything just disappeared; those were also older plasma. Those were really annoying times, heh. That is not a technology that I miss at all.
    – Jason C
    Feb 22 at 17:35
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    As a former PLATO user, I don't remember anything weird about the screen. Feb 22 at 18:43
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    @JasonC the narrow viewing angle was in fact a security feature keeping plasma screens a thing over all the years. No unwanted peeking from the sides.
    – Raffzahn
    Feb 22 at 22:22

1 Answer 1

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But apparently a plasma screen has the remarkable property that it does not need refresh, i.e. it doubles as its own video memory.

This depends of course on design, as there are many ways a plasma screen can be built and operated. For example, plasma TVs have next to no persistence. But for terminals a different approach would be needed - one that uses the ability to use a higher voltage to ignite a cell and a lower one to keep it lit.

A plasma display is basically the same as a fluorescent lamp. Like these it needs a high voltage to ignite, but a much lower voltage to keep 'burning' (*1). Except, it's not one fluorescent lamp, but an array of thousands.

For a plasma display, operation is done by a grid of horizontal and vertical wires located on front and rear of the display, crossing over each cell.

To ignite:

  • a positive voltage is applied on one horizontal line,
  • a negative voltage is applied on a vertical line,
  • both a bit more than ignition voltage;
  • the cell where both cross will light.

To keep a picture displayed, all lines will get a voltage applied a bit over half the holding voltage, resulting in all ignited cells continuing to create light, all othern staying dormant.

And yes, this only needs storage for a single drawing command at a time. In addition the commands can be sent in any sequence, as it's fully pixel addressable. Except for the clear command, that is :))

Just how long was the image on a 1970-era plasma screen stable for?

As long as the hold voltage was applied.

And for a few years, that was a key advantage.

There were more advantages that kept them being available even when memory became low cost. As already mentioned, it allowed extremely high resolutions (at the time) without large refresh memory. Plasma was for example used for high security terminals, as the viewing angle was rather narrow (*2) while the display was at the same time quite bright within that angle, allowing daytime usage without anyone peeking in from the sides. A typical use case would be in banks.

In fact, plasma terminals used by banks well into the 1980s when they were equipped with full memory, so they can operate exactly like regular terminals, but keeping the display advantage.

Similar for military/security usage - like in embassies, as they avoid a common attack channel: due their static nature they do not emit any repeated RF signal that could be captured to 'look' at the screen (*3).

I'm trying to reason about what it would be like to use such a system. [...] You would always have to redraw the display based on internal data

That's much like any other terminal using persistent storage - think Tektronix 4010 et.al.

so it would be natural to design around 'page at a time' instead of continuous scrolling.

Which for most parts wouldn't be a big deal. Scrolling was a non-issue for most mainframe applications anyway, as they usually operated in block mode.

The only real restriction compared with terminals with local storage would be that it was impossible to redraw just a portion of the screen. Due the way persistence is created, it can not be switched off for selected points/areas. (*4)

just because the display doesn't need refreshing every 1/60 of a second, doesn't necessarily mean it persists forever.

It will as long as hold power is applied.


*1 - It's the filling of Neon (for light conversion), Mercury (for energy conversion) and Nitrogen (creating a useful hysteresis).

*2 - It took quite some time to widen the viewing angle to make plasma TV a useful application.

*3 - A measure found only at embassies of rather wealthy nations.

*4 - If at all for whole horizontal or vertical regions, so it might be possible to clear for example the lower half, but I do not know if that was ever used.

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    Could plasma display contents be corrupted by things like the light from a camera flash? By my understanding, neon bulbs' striking voltage can be effectively reduced by the presence of light.
    – supercat
    Feb 21 at 22:37
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    @supercat Never noticed anything alike. While theoretical possible, it may still be a hard to produce. it would need a lot of light to do so.
    – Raffzahn
    Feb 21 at 22:49
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    @rwallace Well, yes, except the drawing commands were more complex, like set position, draw string, draw line and so on. None of those needed much memory, beyond some registers to keep track of a line or an area that is. As said, technical clear was only possible for a whole horizontal or vertical area, but I belive it was only done as whole screen. Further, mouse wasn't an issue, it was touchscreen. Like modern tablets don't do a mouse cursor - after all, ones finger already points where the touch will happen. No need for a mouse pointer as a crutch :)
    – Raffzahn
    Feb 21 at 23:26
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    @rwallace I would be careful about the erasure part. Lowering the voltage on one axis will lower it for all pixels on that row or column, carrying the risk to loose more than just the desired pixel. As mentioned, there are many ways to create plasma displays, so unless the cited article is explicit about the PLATO IV display (that's the one from the 1960s, the later are of different design) , id' still be sceptical about deleting a pixel. Not to mention that deleting and overdrawing is a way too complex operation software wise. A look at the command language might help as well.
    – Raffzahn
    Feb 22 at 0:47
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    @Raffzahn: I would think it would almost certainly be possible to design a display which could handle individual pixel erasure, but displays that don't need to accommodate that could be a lot cheaper. Further, I'd expect that erasing a few complete rows and redrawing them could often be faster than trying to erase pixels individually, since the time to reliably extinguish an arc is longer than the time to establish one.
    – supercat
    Feb 22 at 13:58

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