The Stored Dot

1948 · Charge Well · Work 608 of 1,871

The Stored Dot, 1948, Charge Well

Extreme close-up on the phosphor surface of a storage tube, a single glowing dot at centre surrounded by a dark ring of displaced charge like a well, the phosphor grain visible as fine speckled texture across the whole frame, the glow of the dot fading in a soft gradient toward the dark ring, faint neighbouring dots softly out of focus at the edges of the frame, the surface of the glass filling the frame edge to edge, the dot itself the single light source, its glow falling off into pure black.

What it was used for
The stored dot was the single bit in the cathode ray tube store of the Manchester machines. A short bright-up pulse of the electron beam put a dot at a fixed place on the screen, and that meant 0. A longer pulse wrote a dash, and that meant 1. Reading was not done by eye but through a metal plate in front of the screen that picked up the change of charge capacitively. On a standard six-inch tube the Manchester Baby held 32 lines of 32 digits each, that is 1024 dots. The machine worked serially, one digit every 8.5 microseconds. According to the account by Williams, Kilburn and Tootill it was completed in June 1948 and existed mainly to test this store.
What was new
A bit no longer needed a component of its own - not a tube or a relay per digit, which the delay line had also avoided - but was a place on a surface, reached by a deflection voltage. An extra bit therefore cost almost nothing, and the address was a voltage. What shape the patch should have was found by trial: the Manchester group tested dot-dash, dash-dot, focus-defocus and defocus-focus, and the early machines ran the dot-dash method. The catch lay in reading. Scanning a dash charged its dot position positive again, so reading destroyed the value and forced an immediate rewrite. The dot existed only as long as the machine kept touching it again.

Connections

Every connection names its kind. “Forerunner” on its own would be a claim.

  1. Works withWilliams-Kilburn Tube1947The bit is a spot of charge on the tube's phosphor screen, written by the "dot-dash" or "defocus-focus" method that Kilburn set out in an internal report in 1947. A metal pick-up plate on the outside of the screen senses the charge at each spot and so yields the value of the bit.Source: History of the Williams-Kilburn Tube - University of Manchester, digital60 archive · The Baby - Mark 1 Story, University of Manchester computer50 archive

Further reading