Question: I stumbled on a retro Dutch [video](https://youtu.be/J-gCBh6rk3M?t=11) from 1983 where a large four color mechanical display is shown.
Each pixel consists of a cube which can be turned in order to change the pixel color between 4 colors (white, green, red and blue), a carriage moves behind the screen and slowly updates the screen by altering the rotation of the cubes. Ah this retro electro-mechanical stuff, I love it.

**My question is: how does the update carriage know the rotational position for each of the pixels?**

Or is the rotational state not obtained but reset in some way? The pixels have small cams on their sides, but they seem similar for each color, I can't really see it properly. When the carriage passes it performs only one color-shift per line (90 degrees), and each pixel is always cycled during an update, even for cases where there is no difference between the initial color and target color of the pixel.

Other details:
The displays seems to be branded "AVTEL". And seems to consists of 3 smaller displays next to each other.

[![Screenshot of the video showing the pixels](https://i.stack.imgur.com/VbhQ7.png)](https://i.stack.imgur.com/VbhQ7.png)
[![Screenshot of "AVTEL" four-color mechanical display](https://i.stack.imgur.com/94BD0.png)](https://i.stack.imgur.com/94BD0.png)

Answer: 
Your board outputs logic levels like +5V or 0V to control the LEDs, these are voltage, and voltage is relative, it is the difference in electric potential between two points. You can't have a voltage if you have only one wire, you need two. One of them is often called "ground" or "0V" and it is used as a voltage reference relative to which all other voltages are measured.

If you don't connect grounds, then the receiving chip on the other end has no way to know about that reference, so it will have no idea what voltage it is receiving. So it won't work.

> 
> can the connection to the data input on the Neopixels from the SBC potentially be dangerous in this scenario?
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> 
> 

If you don't connect the grounds together then both devices will be at random potential relative to each other, which means that if current has to flow it will flow via what's connected, which is the IO pins of your chip, instead of just through copper. This is when it fries.

You would worry about connecting grounds if they were at different potentials, for example if you have long cables between devices, and these devices are earthed to different places in different buildings. Also if both devices are earthed in a way that creates a long enough loop of Earth wire in the building that is closed by your ground connection, because when lightning strikes in the vicinity it will induce current into conductive loops. This is why Ethernet uses isolation transformers, for example.

But that would not be the case if you use two power supplies powered from the same socket. No worries in your case.
Connecting the grounds between two systems that are in physical close proximity to each other is normal and done frequently.

If however your systems are far apart (on the order of 20+ ft.) , then you'll want to use some sort of isolation system to allow them to communicate while maintaining separate grounds.