Built a 1975-era Computer (sort of)

In late 1974 I was fortunate enough to get some experience using an HP-2000F minicomputer with a teletype terminal in my school. Compared to using a computer today, the teletype was noisy and slow, and seemed to self-destruct every few weeks, needing repairs or adjustments to keep it printing clearly. Still, the experience seemed magical to me. A friend and I would play various games, including a Star Trek game that was purely text-based, but kept us engaged for hours as the teletype slowly printed the outcome of every Klingon battle.

I didn’t realize it then, but this was during the rise of the “minicomputer”. Minicomputers were smaller than the room-sized mainframes and cost much less, but were still the size of a refrigerator or two. Minicomputers were made by several companies including IBM, Digital Equipment Corporation (DEC), Prime, Wang, Data General, and many more. Corporate departments, mid-sized companies, and universities all found uses for the cost-effective minicomputers. In 1983, my employer’s financial systems ran on a DEC PDP11. I programmed using the DIBOL language to create sales or financial reports.

Fast-forward to modern times, when I stumbled upon information on the internet about a fellow in Switzerland who sells 6:10 scale reproduction kits of the Digital PDP-11/70 front panel, complete with a row of 1970s-era red and magenta switches and the requisite “blinkenlights” flashing away. The kit runs hardware emulation on an embedded Raspberry Pi computer. This was perfect, since I had an unused Raspberry PI in my stock of spare parts. The kits are built to order, so I ordered one. The kit arrived a few weeks later.

The parts came in several bags, one with switches and the other with everything else. An acrylic panel faithfully replicates the colors and logo of the PDP11/70, along with a display case and rear panel to house the electronics and any interfaces the builder opts to include. Since the Pi includes WiFI, I’ll likely let this computer connect wirelessly, but I still included an RJ-45 ethernet port on the back to allow for a hard-wired network connection.

The kit took about 6 hours to assemble. Included were 30 switches of various types, 64 LEDs, 37 diodes, 2 rotary encoders, 18 resistors, and an 8-channel logic driver integrated circuit. I was pleased that everything worked when I powered up the circuit board the first time. After the PDP11 replica was assembled, I added it to my network and logged in.

I booted into BSD 2.11, an early UNIX variant, and explored the file systems. Under the /usr/games directory were programs called trek and hangman, both of which I used almost 50 years ago. I typed ./trek to start the program and instantly remembered the game’s commands and strategy. It all came back to me – surprising, since I usually can’t remember what I had for breakfast on any given day. The main difference was that I was using a terminal emulator, which instantly displays the text output where the older teletype would take forever to type each character at a blazing 110 baud communication speed, adding to the sense of anticipation.

This PDP11 replica is mainly a novelty, but I’ve enjoyed logging onto it, booting various operating systems, compiling old software, and navigating the file system using commands from the very origins of the UNIX operating system. What a great way to revisit childhood memories from a time when computers were not yet in our homes.

About Kevin Forth

Always learning, Kevin is an IT professional that likes to tinker with electronics, motorcycles, and whatever he can take apart.

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