Michael P. Turns to a Raspberry Pi Pico to Reverse Engineer These Classic Plessey Micro-LED Displays

Having tried to figure out how these DIP-format displays work by hand, the problem was handed over to a clever automated testing rig.

Gareth Halfacree
1 month agoDisplays / Retro Tech / Lights

Maker Michael P. has made it easier to add some vintage display flair to your next project — by reverse engineering classic Plessey GPD340 5×7 two-character bitmap display modules, so you don't have to.

"Last year I came across and bought myself a bunch of these cute little displays," Michael explains of the glowing DIP-format devices. "After fruitless attempts to find any information on them, I decided to try reverse engineer the pinout and driving scheme in order display something on them. This has turned out to be successful."

Developed by British electronics firm Plessey in the 1980s to provide an easily-readable display for equipment requiring a custom character set, each Plessey GPD340 module takes the form of two 5×7 LED matrices side-by-side in an DIP package. With just eight pins to drive 70 individual LEDs, though, it's clear that the parts need something clever driving them — the only question: what?

"Given the low count of pins (just eight)," Michael explains, "it was assumed that the display has to utilize some serial protocol to load pixel data into internal registers. Thus, it should have at least one serial clock input and at least one serial data input and the other pins could be chip-selects, output-enables, etc."

Initial attempts to figure out the controlling pins and format of the serial protocol by hand failed, so Michael built an automated test rig powered by a Raspberry Pi Pico microcontroller board and a Texas Instruments INA219 current sensor. "After a few minutes of running, some random pixels on the GPD340 begin to glow dimly," Michael says, "which was a good sign."

"Then," the maker continues, "after a full run and analysis of the collected current monitor log — it was clearly visible that combination of pins caused the current to surge. After that, it was very easy to see which pin is which. Further figuring out the serial protocol was done by manual experimenting on the same breadboard."

The result is a pin-out and driving scheme, published to GitHub under the permissive MIT license, which allows anyone to make use of the same displays — even extending to multiple units, as Michael has proven with a six-module scrolling display.

More information is available on Hackaday.io.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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