Nicholas Bodley wrote:

> [about spinners]

PLATO's text display had three modes: write (turn pixels on,
additively), erase (turn pixels off) and rewrite (replace a block of
8x16 pixels). It also had escape codes to switch modes within a string;
and this unsupported feature was applied to make many animations in
groupnotes.

> (At least, I think every character involved is ASCII.)

PLATO's character set had some oddities.

--
Anton Sherwood, http://www.ogre.nu/
"How'd ya like to climb this high *without* no mountain?" --Porky Pine