Talking to the pixels
SPI, DMX, E1.31. Bringing order to the chaos
This has geekfest written all over it.
The PlanetChristmas Magazine staff was told to build a pixel tree. As the whiteboard filled up with things to figure out, one item was circled in red and my name was written next to it: How do we control the pixels?
I was told to become an expert in pixel communication. That’s when Chuck Smith edged into the room, read the to-do list and said “keep the geekiness to an absolute minimum when writing about it. My mother is going to build one of these things.” Easy for him to say as he sits in that fancy chair behind the giant publisher desk.
I started digging and found out the only thing apparently in common with all the different pixels available out there is the electrical interface. Someone said they were SPI based and that was the hint I needed to get pointed in the right direction.
SPI is the Key
With a little help from Wikipedia I found a definition: the Serial Peripheral Interface or SPI bus is a synchronous serial data link, a de facto standard, named by Motorola, that operates in full duplex mode. It is used for short distance, single master communication, for example in RGB based pixel ribbons.
As I checked around with engineers, that SPI acronym kept popping up in the conversation. SPI is the key.
I started looking at the Light-O-Rama Cosmic Color Ribbon since it has been around from 2009. They do all the hard work for us by integrating the pixel ribbon and controller seamlessly into their communication network as well as their sequencing software. Although Light-O-Rama doesn’t come out and say it, I’m betting that controller uses a form of SPI to control the pixel ribbon. The Light-O-Rama software ‘talks’ to the Cosmic Color Ribbon controller via LOR’s proprietary protocol or DMX-512.
What’s DMX-512? Chuck said I had to keep the geekiness to a minimum but I can say it’s how all those fancy lights are controlled at the live stage shows. Think major lighting eye candy at Trans Siberian Orchestra performances.
DMX to SPI
Soon I found a gizmo from HolidayCoro.com called the ‘DMX to SPI Smart Pixel Decoder’ and I sensed I was on the right path. Sure enough, plug a standard DMX signal into one side and the other side can talk to just about any pixel device out there. Check around and you will find all sorts of devices to do this for you.
Most sequencing programs provide a way to communicate with DMX devices through a USB port on the computer. This is now starting to make sense.
Seems DMX has a limitation. It can only handle 512 control channels. That seems like a lot until you realize each pixel needs three channels (red, green, blue). Some quick math shows DMX-512 will control 170 pixels with a couple of channels left over. The pixel tree we want to build has 1200 pixels so we’re going to need a lot of these ‘DMX to SPI’ decoder thingies meaning we’re going to need a lot of USB ports on our computer.
I started to call my engineering buddies again and someone said ‘E1.31’ would solve all known problems in the galaxy. It’s a way to send lots of DMX information in a hurry over a standard computer network. Think Ethernet.
DMX still limits you to 512 control channels. The pros call this a universe. If you need to control more than 512 channels, then you need more than one DMX universe. Ends up this i
s quite common in the lighting world.
DIYLEDExpress.com has a board that plugs into your LAN and converts control signals from your sequencing software into six separate DMX-512 universes. Now I don’t need a USB port on my computer for each DMX-512 universe.
Just when I was feeling confident this was the direction to go to control our pixel tree another engineer said there was a way to bypass the little ‘DMX to SPI Smart Pixel Decoder’ board to save a little money. Seems there are several controllers that connect to E1.31 and output SPI information directly to pixels. Now you plug the pixel ribbon or string directly into this board and you can control 12 or more pixel devices. These boards even have native DMX ports so you can still use your DMX to SPI Smart Pixel Decoder boards if you want, meaning a single controller can handle 2000+ pixels.
Luckily there’s another article in this
edition of the magazine that goes into more detail about connecting the Advatek Lighting Pixlite E1.31 board to your pixel strings. We also show how to connect pixel ribbons directly to a Joshua One Systems ECG-P12S E1.31 board in still another article. My head is hurting from all this geek talk.
I’ll admit this is some super-geeky stuff I’m trying to simplify enough for my mother to understand. Is it worth the headache? Wait until you see a pixel tree in action. Trust me, it’s worth the hassle.
This article was included in the April 2014 issue of PlanetChristmas Magazine.
By S. William Hawking