PiDP-11

The PiDP-11 is a modern replica of Digital Equipment Corporation’s influential PDP-11 minicomputer. Before, we had a computer on every desk and in every home–and long before they were in every pocket–computers were large, intimidating, and locked in special rooms at universities and corporations. The PDP series paved the way for making computers more accessible.

PDP stood for Programmed Data Processor, and this distinguished line of computers was produced from 1957 to 1990. Each PDP model was numbered sequentially from one to sixteen (skipping PDP-2 and unlucky 13). The PDP-1, PDP-8, and PDP-11 are best remembered today. The PDP-11 was DEC’s first 16-bit computer and cost $20,000 when released in 1970. Reportedly, 600,000 units were sold over its long life, and it was the tool of choice for countless computing innovations, including the creation of UNIX and the C programming language. The 32-bit VAX minicomputer eventually replaced the venerable PDP-11.

This PiDP-11 project was a birthday present to myself. I have long revered the early “big iron” systems that spearheaded the Information Revolution. I was eager to have a symbol of that influential era in my collection.

Oscar Vermeulen is the creator of this innovative PiDP-11 kit. There is no store; you add your name to a waiting list for future shipments. I didn’t have to wait long before he emailed, saying another run was on the way. After paying and waiting for the package to arrive from Switzerland, I was ready to assemble!

I was excited by this kit for several reasons: first, I wanted a good-looking PDP-11 replica; second, I wanted to bask in the glow of blinkenlights; third, I was eager for my first Raspberry Pi project; and finally, I needed to hone my soldering skills. I was able to achieve each of these in reverse order.

Following Oscar’s instructions and after watching a series of walk-through videos, I was ready to start with the passive components. After placing and taping the diodes and resistors in place, I flipped the board and started soldering. Up to this point, I had handled both through-hole and surface-mount projects, but always in small numbers. This project allowed me to spend quality time with my iron.

For some reason, my kit was missing the 330-ohm resistors. I had resistors close to that value in my stash, but I wasn’t sure how important it was to match the designed resistance. After looking at the board, it was clear the resistors were connected to the LED lights. Thanks to help online, I learned the resistance level could vary within a reasonable range, though it might affect the brightness of the lights. I decided to take the opportunity to expand my in-house supply and ordered a variety of kits in different sizes, including the 330s I needed.

Next, I followed the instructions, placed the fiddly spacers on the LED lights, and aligned them on the PCB using the provided guide. Next, I installed the pair of rotary encoders, the chip socket, and the Pi connector (on the back).

I was now ready to test my work. This involved setting up the Raspberry Pi. There is one electronics shop remaining in my area, so I stopped by to see what Pi-related gear they offered. Due to what I assumed was a pricing error (which I brought to their attention), I could pick up a Pi 3 Model B for less money than a stock Pi 3. Thankful for my good luck, I also purchased a suitable power supply and a 16GB memory card.

The Raspberry Pi website makes it very easy to install the Raspbian OS on the memory card and get the Pi up and running. Next, I downloaded the files needed to emulate the PDP-11 from the link provided by Oscar. After reviewing the testing process in the documentation, I felt good about my progress.

Next, I needed to wrangle switches. The instructions make it evident that some struggle to align the switches correctly. I did not have too much trouble but was prepared for difficulty. There is both a lower and upper guide. The lower guide helps you put the right switch in the right place. The upper guide helps align the spacing of the switches so they aren’t cockeyed once soldered into place. Zip ties are used to squeeze the guides together to fit everything into place.

Finally, I turned my attention to the case. The case and front panel are well-made and provide a great fit and finish to the kit. I struggled to get the board and faceplate properly screwed into the case. The combination of spacers, nuts, and bolts was more troublesome than I expected. But once everything was aligned, it looked great.

By default, the front key switch is not connected, but the board is configured so the key can either turn off the 5V power or issue a software command to shut down the Raspberry Pi. Not wanting to make a permanent decision, I soldered two wires to the key switch and installed screw terminals at both locations on the board. I then placed a barrel jack on the side of the case and installed another screw terminal for the 5V power, bypassing the Raspberry Pi’s micro USB power connection.

First, I played around with the software shutdown option. It took some trial and error to figure out which way the key needed to turn to operate correctly, but once you flip down the Halt switch, you turn the key to issue the proper “shutdown -h now” command before unplugging the Raspberry Pi. However, I preferred to use the key to cut power to the board-mounted barrel jack. This way, I could use the PiDP-11’s “secret” shutdown switch enabled by pressing the rotary encoder and then cut power to the Pi by turning the key. Also, it is satisfying to use the key to power the unit. Since the Pi automatically boots into the blinkenlight display, operating without an attached keyboard and screen is easy.

I did attempt to install panel mount connectors to the back cover extending the Raspberry Pi’s power, USB, HDMI, and Ethernet ports, but there was not enough clearance for the cover to close properly.

I’m not done yet. I may enable a serial terminal connection for the PDP-11 emulator, and I will continue exploring the PDP and Raspberry Pi software. The point of any hobbyist kit is to tinker and learn. I can do both with this well-made piece of ’70s computing nostalgia.