The RCA Selectron -- Selective Addressing

This is a very simple simulation of one method of writing individual bits into a Selectron.
(150 kb java applet)

The previous simulation explained how an electron beam can interact with a storage eyelet. Writing pulses are applied to the eyelets by capacitive coupling to a backing electrode. A problem is that all eyelets electrically floating; i.e., they are all capacitively coupled to one electrode. This would seem to be a major problem for selecting only one eyelet is necessary -- a 256-bit memory is sub-optimal if all 256 bits are always identical. It is necessary to devise a means to selectively charge or discharge an eyelet.

If the pulse is shared by all eyelets, then the only remaining choice is to selectively control the stream of electrons. If an eyelet is pulsed with no electron stream bombarding it, its charge or lack thereof will not be changed. By blocking electron flow to all but the desired eyelet, selective writing can be accomplished.

Just as in the previous simulation, the user can choose to pulse the eyelets high or low. But in this model, the user decides which eyelet -- or storage location -- is addressed but turning off the electron flow to all other eyelets. This simulation also disables the cathodes for the length of time needed to charge or discharge the eyelet*.

Applet source code
Applet built with Processing
*OK, how many times is this guy going to use the word "eyelet" already? "Blah blah blah eyelet eyelet eyelet...." Fair 'nuff. First, the storage thingy in the planar type Selectrons is an eyelet. Secondly, the eyelet has a major role in the history of electron tube production: tubes were built like shoes. Really.

Shoe manufacturing in the New England region of the United States was a major industry in the early 1900s. Automation of shoe assembly was needed to keep costs low while producing large quantities of shoes. Shoes of many styles and sizes. Flat materials of leather and cloth needed to be cut to the proper sizes. Some materials needed to be bent or formed to odd shapes. They needed holes to be punched in the materials. They needed to be held together in alignment while being assembled. And if the shoes had laces, they needed eyelets for the laces to pass through. Well, building electron tubes required pieces of metal and mica cut to the desired size and shape, holes to be punched and needed them to be held together for assembly. Sounds as if some automation equipment may be already have been developed for automation of tube assembly.

And a major designer and builder of automation for assembly of electron tube innards was a company known as "USM." Yup, United Shoe Machinery. So, look down at your shoes, notice the eyelets the laces pass through, then look back at the images of the Selectron. See all the stamped layers. You may not be able to see the storage eyelets buried inside, but you'll see the small eyelets holding the layers together. Your sneakers and Selectrons share a common heritage.