Pico Driver for Plasma Self-Scan Displays!
Reading about this project sent me down a rabbit hole of Googling, patent reading, and generally being fascinated. This Pico-based driver for plasma self-scanning displays is an awesome project for making use of this vintage display tech. What is a plasma self-scanning display? I’m glad you asked!
In the 1970s, Burroughs (yes, that Burroughs) saw the rise of computerized terminals which invariably used CRTs. CRTs are okay, but for applications like point-of-sale systems they have a few disadvantages: they’re bulky and heavy, they have a relatively limited lifetime, they can suffer severe screen burn-in if used with semi-static displays (which is pretty much all POS machines display), and they required maintenance (if you cared enough). Burroughs had developed a new kind of plasma display that made the drive electronics quite simple, for the day. Instead of a fully multiplexed display, only the row drivers were driven. The columns were scanned with a three-phase clock, which would “push” each lit segment as the rows ionized the plasma, hence the “self-scanning” name.
They had other advantages, too: plasma displays are extremely sharp and high-contrast and are readable in all lighting conditions. Burroughs advertised them as not needing the “dangerous” high voltages of CRTs, instead only needing a “medium” voltage… of -250VDC! Not exactly low-voltage, but that voltage was needed just to start the ionization. Once the plasma was created, it could be maintained with much lower voltages as the plasma is conductive. Like many technologies of the time, the Soviet Union made compatible clones, which are generally easier to get a hold of than the original Burroughs displays, which are expensive and difficult to find.Back to the Pico driver board — this takes care of interfacing the Pico W to the “medium” voltage of the plasma display, as well as actually driving the display.
The developer has used one of the Soviet single-line versions to create an in-car temperature display. The beautiful, warm neon glow is extremely readable and adds a retro look to the interior! However, you can quickly adapt the firmware to display just about anything you want. The displays aren’t actually bit-mappable, as far as I understand; they have built-in characters sets (commonly ASCII, Cyrillic, and Hebrew) which again makes driving them even easier for microprocessor boards of the 1970 right up to today’s microcontrollers.
So whatever happened to this technology? Well, Burroughs advertised the main advantage of being simple drive electronics. Fully multiplexed LED displays took a huge number of pins to drive. But of course, custom driver ICs quickly swept the market and LED/LCD display technology dominated because of the low-cost and high reliability. If you want to learn more, check out this Twitter thread by @foone!