How long did plasma displays persist?

Question

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?

Answer 1

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.

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*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.