Storage tube

TheTektronix 4014uses a storage tube for its display.

Storage tubesare a class ofcathode-ray tubes(CRTs) that are designed to hold an image for a long period of time, typically as long as power is supplied to the tube.

A specialized type of storage tube, theWilliams tube,was used as amain memorysystem on a number of earlycomputers,from the late 1940s into the early 1950s. They were replaced with other technologies, notablycore memory,starting in the 1950s.

In a new form, the bistable tube, storage tubes made a comeback in the 1960s and 1970s for use incomputer graphics,most notably theTektronix 4010series. Today they are obsolete, their functions provided by low-costmemory devicesandliquid crystal displays.

Operation

Background

A conventional CRT consists of anelectron gunat the back of the tube that is aimed at a thin layer ofphosphorat the front of the tube. Depending on the role, the beam ofelectronsemitted by the gun is steered around the display using magnetic (television) or electrostatic (oscilloscope) means. When the electrons strike the phosphor, the phosphor "lights up" at that location for a time, and then fades away. The length of time the spot remains is a function of the phosphor chemistry.

At very low energies, electrons from the gun will strike the phosphor and nothing will happen. As the energy is increased, it will reach a critical point,$V_{cr1}$,that will activate the phosphor and cause it to give off light. As the voltage increases beyondV_cr1the brightness of the spot will increase. This allows the CRT to display images with varying intensity, like a television image.

AboveV_cr1another effect also starts,secondary emission.When any insulating material is struck by electrons over a certain critical energy, electrons within the material are forced out of it through collisions, increasing the number of free electrons. This effect is used inelectron multipliersas found innight visionsystems and similar devices. In the case of a CRT this effect is generally undesirable; the new electrons generally fall back to the display and cause the surrounding phosphor to light up, which appears as a lowering of the focus of the image.

The rate of secondary emission is also a function of the electron beam energy, but follows a different rate curve. As the electron energy is increased, the rate increases until it reaches a critical threshold,V_cr2when the number of secondary emissions is greater than the number supplied by the gun. In this case the localized image rapidly fades as energy leaving the display through secondary electrons is greater than the rate it is being supplied by the gun.

In any CRT, images are displayed by striking the screen with electron energies between these two values,V_cr1andV_cr2.BelowV_cr1no image is formed, and aboveV_cr2any image rapidly fades.

Another side effect, initially a curiosity, is that electrons will stick to the phosphor in lit up areas. As the light emission fades, these electrons are likewise released back into the tube. The charge is generally far too small to have a visual effect, and was generally ignored in the case of displays.

Storage

These two effects were both utilized in the construction of a storage tube. Storage was accomplished by striking any suitably long-lived phosphor with electrons with energies just aboveV_cr1,and erased by striking them with electrons aboveV_cr2.There were any number of varieties of mechanical layouts used to improve focus or cause the image to be refreshed either internally to the tube or through off board storage.

The easiest example to understand are the early computer memory systems as typified by theWilliams tube.These consisted of World War II surplus radar display CRTs connected to a computer. The X and Y deflection plates were connected to amplifiers that converted memory locations into X and Y positions on the screen.

To write a value to memory, the address was amplified and sent to the Y deflection plates, such that the beam would be fixed to a horizontal line on the screen. Atime base generatorthen set the X deflection plate to increasing voltages, causing the beam to be scanned across the selected line. In this respect, it is similar to a conventional television scanning a single line. The gun was set to a default energy close toV_cr1,and the bits from the computer fed to the gun to modulate the voltage up and down such that 0's would be belowV_cr1and 1's above it. By the time the beam reached the other side of the line, a pattern of short dashes was drawn for each 1, while 0's were empty locations.

To read the values back out, the deflections plates were set to the same values, but the gun energy set to a value aboveV_cr2.As the beam scanned the line, the phosphor was pushed well beyond the secondary emission threshold. If the beam was located over a blank area, a certain number of electrons would be released, but if it was over a lit area, the number would be increased by the number of electrons previously stuck to that area. In the Williams tube, these values were read by measuring thecapacitanceof a metal plate just in front of the display side of the tube. Electrons leaving the front of the CRT hit the plate and changed its charge. As the reading process also erased any stored values, the signal had to be regenerated through associated circuitry. A CRT with two electron guns, one for reading and one for writing, made this process trivial.

Imaging systems

The earliestcomputer graphicssystems, like those of theTX-2andDEC PDP-1,required the entire attention of the computer to maintain. A list of pointsstored inmain memorywas periodically read out to the display to refresh it before the image faded. This generally occurred frequently enough that there was little time to do anything else, and interactive systems likeSpacewar!were tour-de-force programming efforts.

For practical use, graphical displays were developed that contained their own memory and an associated very simple computer which offloaded the refreshing task from themainframe.This was not inexpensive; theIBM 2250graphics terminal used with theIBM S/360cost $280,000 in 1970.

A storage tube could replace most or all of the localized hardware by storing the vectors directly within the display, instead of an associated local computer. Commands that previously caused the terminal to erase its memory and thus clear the display could be emulated by scanning the entire screen at an energy aboveV_cr2.In most systems, this caused the entire screen to quickly "flash" before clearing to a blank state. The two main advantages were:

• Very lowbandwidthneedscompared tovector graphicsdisplays, thus allowing much longer cable distances between computer and terminal
• No need for display-localRAM(as in modern terminals), which was prohibitively expensive at the time.

Generally speaking, storage tubes could be divided into two categories. In the more common category, they were only capable of storing "binary"images; any given point on the screen was either illuminated or dark. TheTektronixDirect-View Bistable Storage Tubewas perhaps the best example in this category. Other storage tubes were able to storegreyscale/halftonedimages; the tradeoff was usually a much-reduced storage time.

Some pioneering storage tube displays wereMITProject MAC's ARDS (Advanced Remote Display Station), the Computek 400 Series Display terminals (a commercial derivative),which both used a Tektronix type 611 storage display unit, and Tektronix's4014terminal, the latter becoming ade factocomputer terminal standard some time after its introduction (later being emulated by other systems due to this status).

The first generalized computer assisted instruction system,PLATO I,c. 1960 onILLIAC I,used a storage tube as its computer graphics display.PLATO IIandPLATO IIIalso used storage tubes as displays.

See also

• Direct-View Bistable Storage Tube(DVBST), Tektronix's implementation of a Storage tube
• Cathode ray tube(for an explanation of how analog storage tubes worked)
• Williams tubeandSelectron tubeboth used the term "storage tube" for early computer memory devices
• Electronic paper
• Tektronix 4050used a storage tube to eliminate the need for framebuffer memory