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From: agn@unh.cs.cmu.edu (Andreas Nowatzyk)
Newsgroups: comp.arch
Subject: Re: An old fashioned memory technology, CRT's, how'd they work?
Message-ID: <1010@unh.cs.cmu.edu>
Date: Tue, 14-Jul-87 13:09:46 EDT
Article-I.D.: unh.1010
Posted: Tue Jul 14 13:09:46 1987
Date-Received: Thu, 16-Jul-87 06:16:19 EDT
References: <602@madvax.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 20

The Williams tube uses the same principle that is used in storage scopes
and some old Tektronix graphics terminals: The screen uses a phosphor with an
extremly high resistance, essentially an array of isolated electrodes.
This screen is flooded uniformly with low-energy (say 400V) electrons.
Due to the emission of secondary electrons, each point of the screen can
be in (at least) 2 stable states: a) dark: charged to >= +400V so that
no flood-gun electrons can reach the point due to electrostatic repulsion.
b) light: charged to a more negative voltage so that flood-gun electrons
reach the point. The potential is stabilized at this point due to the
emission of secondary electrons so the the net current is 0.

You can change the state of each point by using a second electron gun that
is focused and uses a higher acceleration voltage (say 2KV). It is easy to
see the a->b change, but the reverse is also possible by playing tricks
with the screen and flood-gun potentials. Readout is done by scaning the
screen with a low-intesity beam and detecting the change is the secondary
electron current. Addressing was done with the X/Y deflection of the readout
beam.

  --  Andreas