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From: weaver@prls.UUCP (Michael Gordon Weaver)
Newsgroups: comp.arch
Subject: Re: An old fashioned memory technology, CRT's, how'd they work?
Message-ID: <4929@prls.UUCP>
Date: Tue, 14-Jul-87 13:36:10 EDT
Article-I.D.: prls.4929
Posted: Tue Jul 14 13:36:10 1987
Date-Received: Thu, 16-Jul-87 07:06:15 EDT
References: <602@madvax.UUCP>
Reply-To: weaver@prls.UUCP (Michael Gordon Weaver)
Organization: Signetics Microprocessor Division
Lines: 61

In article <602@madvax.UUCP> cw@madvax.UUCP (Carl Weidling) writes:
>
>	Yesterday I bought a book called "Bit by Bit, An Illustrated History
>of Computers", by Stan Augarten,Ticknor & Fields,NY,1984.
>	It seems like a good book from what I've seen glancing through it,
>but, I was reading about the design of the Mark I in Manchester,England
>where it says: "He [F.C. Williams, the project's chief engineer] hit upon
>the idea of employing ordinary cathode ray tubes [to solve the problem of
>internal memory storage]...Their operating principle was quite simple;
>"guns" in the bases of the tubes shot positively and negatively charged
>electrons at the faces of the tubes, thus storing bits in the form of charge
>spots, which, by the way, were quite visible to the eye."
>	Well, I have a problem with "positively and negatively charged
>electrons", but also, with how this can be memory.  How long did the charges
>last?  were they refreshed? How was the memory read after being stored?
>	If anybody out there knows this stuff, I would be appreciative
>if you could enlighten me.  My limited impression is that the book is better
>than this little excerpt would make it appear.
>Regards,
>Carl Weidling

This is from memory, and also my interpretation of what I have read, so it 
may not be correct, but here goes.

The electron beam of a cathode ray tube is of course negative. When an 
electron hits the phosphor on the inside of the screen, it knocks out 
several electrons leaving a positive ions. Eventually, these electrons
are replaced, causing the glow that is seen on the other side of the 
screen.

On the wall of the tube between the gun and the screen is a secondary 
anode used to return to ground excess charge given off by the electron 
beam so that the net charge does not grow too large.

When used as memory, the current to this anode is measured (presumably 
converted to a voltage via an appropriate resistor). When the beam is 
writing on to a positive spot of phosphor, slightly less current will 
be going to this anode, because the positive ions have no valence 
electrons to be removed by the electron beam. So we can tell which 
parts of the screen were written to last time.

The readout is destructive, and so the bits must be re-written each 
pass. The charge lasts for about the same time the phosphor glows, 
which is on the order of 30 milliseconds. I believe the largest
number of bits per cathode ray tube was about four thousand.

I have been unable to acertain how the regeneration is done, since not 
more than a few bits of data could be buffered e.g. by tubes.  One 
possibility would be that the data was being written to one tube as 
it was read from another.  The reports I have read suggest that individual 
bits were written back by the same beam that read them, somehow cleverly 
using the fact that the positive areas are somewhat larger than the beam 
that wrote them, but this may be incorrect.



-- 
Michael Gordon Weaver                   Usenet: ...pyramid!prls!weaver
Signetics Microprocessor Division
811 East Arques Avenue
Sunnyvale, California USA 94088-3409            Phone: (408) 991-3450