One of the rarer pieces of my collection, the Burroughs Self-Scan display is one of the promising early plasma glow discharge technologies to emerge in the seventies. At first glance, it is a straightforward dot matrix array using a neon-based fill gas. On closer inspection, one discovers that there is some very clever engineering going on under the hood. The display panel in the unit I have has 112 columns and 7 rows, giving a total of 784 addressable dots. One would normally assume a great many connections to the plasma tube are required in order to address the elements, but this is where the cleverness comes in.
In order to simplify and condense the addressing circuit, a special characteristic of glow-discharge is exploited in that discharges can be transferred from one element to another with carefully timed pulses. A common example of this pulse-transfer concept is the Dekatron tube, in which a counter is implemented by starting a glow discharge in one element, and transferring it to the next, the next, and so on by a common “clock” pulse input. The tube essentially generates a bucket-brigade logic pattern internally without requiring any external circuitry except that which times the internal process.
In the Self-Scan, every third column is connected together in such a fashion as to allow glow-transfer to occur column by column from one end of the display to the other. A three-phase clock drives pulses into the three connected arrays of columns and drives the glow-transfer process along from left to right. Vertical columns of data are presented to seven electrodes on the left of the display for every cycle of the master clock; the columns working their way rightward and generating a visible display. This is an extremely simplified explanation, as there are actually invisible cavities in the rear of the display for transferring a fresh series of glowing discharges into all elements in order to prime them, and more cavities in the front for actually displaying visible glow. Please read the Theory document for a complete explanation of the entire process!
My particular display was in a pile of neat bits that was being discarded at my local university. It took me many years to eventually find enough information in order to make an interface to it. Thankfully, this unit accepts 6-bit ASCII (capitals only) and basic control sequences and this can be interfaced rather easily with just about any computer hardware you can find. I designed a custom switchmode power supply which accepts 12V DC and generates the required -12V and +250V outputs for the display. (I can’t find the schematic, but I will post it once I do).
I interfaced my Self-Scan to my SDK-85 and wrote a simple series of programs to try it out.
Some photos of the Self-Scan unit.
