Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!lll-lcc!mordor!sri-spam!ames!amdcad!sun!wdl1!bobw From: bobw@wdl1.UUCP (Robert Lee Wilson Jr.) Newsgroups: comp.arch Subject: Re: An old fashioned memory technology, CRT's, how'd they work? Message-ID: <3490004@wdl1.UUCP> Date: Tue, 14-Jul-87 12:06:02 EDT Article-I.D.: wdl1.3490004 Posted: Tue Jul 14 12:06:02 1987 Date-Received: Sat, 18-Jul-87 11:06:25 EDT References: <602@madvax.UUCP> Lines: 42 The only machine I personally saw using WIlliams tubes for storage was the ORACLE (Oak Ridge Automatic Computer and Logical Engine... They probably spent almost as much time on the name as on the rest of the system :-)). The system there had the tubes covered so I can't speak to the visibility of the bits, but based on how it worked they _ought_ to have been quite visible except that the sense electrode over the screen may have been opaque. Fundamentally what happens is this: Steer the beam (using electrostatic deflection plates) to a spot on the tube face corresponding to memory address. Now turn the beam on or don't turn it on: Let's assume turning it on corresponds to writing a 1. There is now a negatively charged spot wherever we have written a 1. (The charge does leak away. I don't know just how fast, but have the impression refresh was not needed as soon as for DRAM chips.) To read, write a 1 at the location. If there was previously a 0, there will be significant beam current charging the spot, while if there was previously a 1 the beam will be diffused by the negatively charged spot it was aimed at. There is a difference in charging current which is sensed by a conductive plate on the outside of the face of the tube, essentially one plate of a capacitor with the charges on the inside of the face being the other plate(s). Whenever you read a 0 you have to go back and "write a 0" since by reading you wrote a 1. To do a periodic refresh you just scan a whole raster and write back whatever you read. Note: I haven't said how to write a 0 in an already charged location, since I can't remember for sure. No, they didn't shoot positive electrons! I _think_ you sort of write a ring of charge around the spot, "smoothing out the charge hill," but I would be interested in anyone who can say for sure. The tubes were only special in that they were carefully selected for freedom from blemishes in the faceplate. As a kid I always dreamed of getting together a bunch of 5BP1's and building a system. I saw the ORACLE when I was 13, and may have screwed up the description somewhat, but I believe that is about right. I remember an early book _The Design of Electronic Digital Computers_, author I think was Richards or Richardson, which had a chapter on this as well as other storage techniques of the day. (Mercury tank audio delay lines for recirculating bit streams, etc.) I haven't seen a copy lately but assume it must still be in some libraries. Disclaimer: The usual copout.