Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rochester!pt!unh.cs.cmu.edu!agn 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