The Connection Machine was a groundbreaking massively parallel supercomputer of the mid-1980s and 90s. Just as incredible as the machine’s performance was its industrial design: an ominous black cube-of-cubes, with system activity conveyed through thousands of red LEDs. It looks straight out of sci-fi…but it’s real!

There are a handful of truly iconic computer designs, like the distinctive “C” shape and built-in bench of early Cray supercomputers, but the Connection Machines are simply the best, period. I’m sorry, this isn’t mere subjective opinion, it’s irrefutable science. Look at it!

Programmed in Python!

If building with a Raspberry Pi, there is example code for three different animations. For the Pico microcontroller board, only the first of these is provided, but it’s the one most folks want anyway…

Parts from Adafruit

Required and/or recommended parts for this build:

The full design requires eight each of the LED drivers and matrices.

The “brains” can be one of:

• Any model of Raspberry Pi 4. The basic 1 GB model is adequate if you just want to run the demo code…or opt for bigger models if you expect to be doing real work. Older Pi models could also work, using USB micro-B instead of USB-C for power.
• A Raspberry Pi Pico microcontroller board.

One or other, you do not need both!

At the time this is being written, Pi 4 supplies are constrained…one could build with the Pico for now, and swap out for a “real” Pi later.

These wires and heat-shrink bits make for a luxurious build…though if you’ve been doing electronics for a while you might already have suitable bits around:

You do not need this Pico RP2040 board if building around a Raspberry Pi 4:

You only need the following right-angle connector if building with Raspberry Pi 4. Pico does not require it.

Using this particular right-angle connector does unfortunately block the HDMI ports; it’s implied that such a Pi would be used “headless,” as in an OctoPrint server. If you require HDMI out, you might find an upward-facing USB-C adapter elsewhere, a super-flexy cable, or more advanced folks might change up the wiring to allow using a 5V power adapter into the Pi’s breakout header (shared with the matrices).

Parts Sourced Elsewhere

A well-stocked local hardware store might carry metric fasteners…but if not, these can be ordered online…

Other Tools and Materials

You will need a soldering iron and related paraphernalia. None of the soldering is especially challenging…but with eight of those matrices, it does require patience.

Wire cutters/strippers, small screwdriver, typical electronics workbench stuff.

Most of the case is 3-D printed, so you’ll need a printer, or access to one. This can be made with basic black filament, or read on for alternatives…

Acrylic Panels

The four LED diffusion panels are slightly rectangular: 62 mm wide by 60.5 mm tall. There are a few ways these can be made:

• The score-and-snap method, using a straightedge and plastic scoring tool or back of an X-Acto blade. Wear eye protection!
• CNC milling.
• Laser cutting. If purchasing the LED diffusion acrylic from a retail source like TAP Plastics, some offer laser cutting services for a modest fee…or some makerspaces have a laser cutter for member use.

Here’s a wiring schematic for the Raspberry Pi version. Refer back to this when getting to later steps:

Color-coded by function:

Notice there are two groups of four matrices each. Each group gets wired to a different I2C bus. Specific pin numbers will be discussed later.

And here’s a similar diagram for Pico RP2040:

Build Steps

Repeat the process above to produce eight such 4-way splitters in total, two each for power, ground, I2C data and clock. You can optionally use a bit more heat-shrink to tidy up the wire bundles.

Assembling the matrix-and-driver sandwich is explained in greater detail in this guide.

Solder Connection headers

Solder four header pins to each matrix. Solder the shorter pins to the driver board. The longer pins face away from the driver board and connect to the female jumper wires.

As mentioned on the Overview page, the USB right-angle connector will block the HDMI ports. This is fine for “headless” use, as with an OctoPrint server. If you require HDMI out, you’ll need to improvise with a different USB adapter or cable.

Glad you’re here! The Raspberry Pi code has all the bells and whistles. This will be discussed in a moment, but first…

System Setup

Basic Raspberry Pi setup is already covered in other guides… so if this is your first time, start there. You can do these initial steps with the Pi outside the case and without the matrices wired up.

Click the gear icon, or press Control+Shift+X (Windows) or ⌘+Shift+X (Mac), to show advanced options. Essential system info (hostname, WiFi network, etc.) can all be configured before the system is even booted the first time…it’s so much easier this way! Tip: remember to also set the Wireless LAN country to get WiFi working.

So suppose at this point you have a Raspberry Pi that’s booted and accessible via an ssh remote session…

Prerequisite Software and Setup

First is to edit the system /boot/config.txt file, which must be done as root via the “sudo” command:

(You can substitute your editor of preference in place of “nano.” I’m strange and actually like vi.)

Append the following two lines to the end of the file. Just copy-and-paste them verbatim, as the syntax is very persnickety:

Save changes to the file. This will enable two “soft” I2C buses on specific GPIO pins. You can use different pins if both the software and wiring are changed to match, but don’t do this unless other hardware you’re interfacing specifically demands it. Most users should stick with the plan.

Extra Steps for the Audio Visualizer Only

This optional part of the project requires the Mini USB Microphone mentioned on the Overview page. If you don’t have this part or don’t want the visualizer, you can skip ahead to “Continue Setup” below.

We’ll do the audio setup before the other software, as it then requires fewer reboots.

Enter these commands:

Answer “Y” when asked about installing packages.

Following installation, one must then edit the system’s ALSA configuration, which is related to sound input and output:

(As before, substitute your editor of preference in place of nano.)

Look for these two lines, starting around line 76:

Change these to:

Save changes to this file and exit the editor.

Continue Setup

With or without the audio visualizer, all installations will require the following sequence of commands (please use the “Copy Code” button and paste into the Pi’s terminal window…some of these are single long lines that might appear wrapped here):

Answer “Y” when asked about upgrading or installing packages.

That last step—the raspi-blinka.py script—will ask to reboot the system when finished. Play along, answer “Y,” and log back into the system once it’s rebooted.

FYI: Blinka is a package for running CircuitPython (microcontroller) code on “real” computers like Raspberry Pi. It allows leveraging hardware-specific libraries for talking to the LED matrices, for instance. Pretty cool. There’s an entire guide if you’d like to learn more.

Download Project Code and Test

All the system configuration is done, and now to fetch the project code:

Start with the simplest of the examples, some animated lights inspired by the CM-5 control room computer in Jurassic Park:

python chaser.py

If all goes well, this will simply run, lighting all 8 matrices with animated bit patterns. Press CONTROL+C to stop.

If you’re seeing a RuntimeError or ValueError message from the system, it’s usually one of these things:

• An SDA/SCL wire pair is swapped, either on the Raspberry Pi header or on one or more matrices.
• SDA and/or SCL are connected to the wrong pin(s).
• Address select pads on one or more matrices are not correctly set; multiple matrices on the same I2C bus are set to the same address.
• A poorly-soldered connection in one of the 4-way splitters.

Remember there are two I2C buses at play here, each with four matrices, each of which should have a unique address on that bus.

For posterity, here’s that wiring diagram again:

From a programming perspective, chaser.py is the simplest of the examples, as it doesn’t tie into any obscure system services…it just animates the lights. If creating your own programs for the Little Connection Machine, this is a good starting point.

Then there’s the classic supercomputer blinkenlights:

python cpuload.py

The amount of pixels lit is proportional to the current system workload. Left running in the background…when compiling and debugging two million lines of code to run a dinosaur theme park, you’ll see this really light up! CONTROL-C to stop.

If you have the USB microphone installed and did the extra setup earlier, you can try the third example:

python audio.py

This will react to music and sounds within earshot…higher frequencies toward the top of the matrices, lower frequencies toward the bottom. CONTROL+C to exit.

If this fails to run, double-check the ALSA configuration that was described earlier. It’s notoriously picky.

If this runs but the image appears jumbled, the LED matrices are arranged in the wrong order. The wiring diagram shows the address for each matrix.

Auto-Starting on Boot-Up

The preferred way to do this nowadays is via the systemd service. Begin by creating a unit file for this service…

Copy-and-paste the following into the new file, editing the “ExecStart” line to reference which of the Python scripts you’d like to run. Can also change the directory name, if you’ve moved or called it something else:

Save this file and exit the editor.

A few commands then enable this new service and will launch it on startup:

To change which program is loaded on startup, just edit /lib/systemd/system/cm1.service and change the ExecStart line, and reboot. No need to repeat the systemctl commands.

If you’d be satisfied with just the blinkenlights part of the project (this is what most people are after!), we can do this with a Pico RP2040 microcontroller board replacing the Raspberry Pi 4. No complex operating system to boot up, it’s even easier to set up and use.

If you know your way around code and hardware a bit, actually any CircuitPython-compatible board with two I2C buses can work. The Pico is super inexpensive and readily available though, so it is used as the reference design.

If this is your first time using CircuitPython on the Pico RP2040 board, please begin with this guide which provides an introduction and download & installation instructions.

So at this point, you should have CircuitPython installed on the Pico board.

To download the code for this project, click the “Download Project Bundle” button below. This brings together some required CircuitPython libraries, so everything is quickly and easily copied to the CIRCUITPY drive.

If you currently have a CircuitPython project on the Pico board, copy the files off to somewhere for safe keeping.

Unzip the project bundle archive, look inside the resulting folder (a couple layers down if needed), and copy the code.py file and lib folder to the CIRCUITPY drive. The drive contents should then appear like so (you can ignore the boot_out.txt file, it’s auto-generated on startup):

If everything’s connected correctly and the code and libraries installed in the right place, you should get the blinkies. And that’s it! Just plug it into USB any time you need it.

If things do not work, you’ll want to open a serial connection to the CircuitPython console (such as with the Mu editor) and see what error messages it’s displaying. This is explained in the introductory CircuitPython guide previously mentioned.

For posterity, here’s that wiring diagram, showing the connections for this board:

If you see a RuntimeError or ValueError message in the serial console, it’s usually one of these things:

• An SDA/SCL wire pair is swapped, either on the Pico board or on one or more matrices.
• SDA and/or SCL are connected to the wrong pin(s).
• Address select pads on one or more matrices are not correctly set; multiple matrices on the same I2C bus are set to the same address.
• A poorly-soldered connection in one of the 4-way splitters.

Remember there are two I2C buses at play here, each with four matrices, each of which should have a unique address on that bus.

“Do as I say, not as I do”

This code does things that would get you an “F” in any programming class, so please don’t look to it as a shining example of anything.

The adafruit_is31fl3731 library normally provides easy X/Y pixel-setting functions and robust exception handling. Any well-behaved code really should be using the functions provided there, as documented in this guide. If you want to display other things on your Little Connection Machine and don’t need super-frequent updates, please program it that way…or build the Raspberry Pi version.

In order to get fast updates on the LED matrices, this project’s code plays games with the adafruit_is31fl3731 library to dump data directly to the matrices, bypassing the aforementioned functions. It relies on the fact that it’s just a random bit pattern, that there’s no coherent order that must be followed to produce an image. Handling that would add inordinate complexity to this code.

Such complexity is something the Raspberry Pi is perfectly equipped to handle… and if you go looking into the Pi code, you’ll see it treats all the matrices as if a single contiguous image, with numerous drawing and image-loading functions available through PIL, the Python Imaging Library. The CircuitPython blinky code is just parlor tricks…and that’s why it’s the only example we provide there.