Decades ago, I was offered a used Decwriter LA30 which was a printing terminal. I wanted hard copy from my CP/M computer of the time. It was a heavy beast; I think the lab that gave it to me was pleased to see it gone.
I also got some technical info with it, probably the same ones archived here. It had a current loop interface, and with a bit of hacking I was able to interface it to RS-232. With that I could type into my computer and print documents. But it got too cumbersome to take with me when I moved home so I stripped the circuit boards out of it and junked the chassis. Retro computing aficionados would be horrified at this, but there was no way I could hang on to the whole machine.
Fast-forward a decade or so, I was searching for something to do with the circuit boards. I noticed that the character generator board had a font ROM, the Mostek MK2002 you see in the picture. This held the 5x7 patterns for the 63 printable ASCII characters. Wait a moment, you say, there are 94 printable ASCII characters! Ah but you see they didn't cater for lower case characters in those OLD SCHOOL LINE PRINTER DAYS, though the keyboard could send lower case characters. (Which may have been part of my reason to abandon it as a printer.)
So I came up with the idea of modifying the circuit on the board to make the counters cycle through the 64 characters and light up 7 LEDs instead of firing 7 printhead pins. The LEDs came from another junk circuit board. Other modifications I made were:
- An astable 2 transistor oscillator to supply the low frequency clock
- A 7805 regulator to drop the input 12V to 5V so that I could power the board from a single power supply
- A charge pump circuit to generate the negative voltage required by the font ROM (the MK2002 runs off +14, 0, -14V lines)
Another thing you will notice in the photo is that the MK2002 came in a white ceramic package. It's not often that you are able to write the pinout functions on the chip itself!
It was something I used to put at the window some nights to mystify the neighbours. Recently I dug this out of my junk box, turned it on and it still worked. So I've decided to post the story.
If you watch the video you might even be able to work out the characters being displayed one vertical column at a time.
This is a very idiosyncratic hack but I posted it to emphasize that even if for one reason or another you don't have access to fancy electronics components, and it gets cheaper all the time, you can still have fun with hardware. All you need is a will to hack, a desire to learn (so easy with an embarrassment of material from the Internet), time, and patience. I've learnt a lot from design notes, schematics of other projects, and datasheets of components. If the electronics are old you might have to search for old datasheets, but there are many charitable retro fans who have scanned in old data books.