Point Source Pixel FAQ / Installation

This post attempts to gather all of the important Point Source Pixel information in one area.  Click on any picture below to enlarge.

For pictures, video clips and more, take a look in the ‘archives’ section and peruse older posts.

Introduction

The point source pixel system was designed in 2008 to be bright, easily chained and easy to use.  To that end, we settled on a system comprised of driver boards, a ribbon cable wiring harness, and small circuit boards containing the RGB LED and appropriate DMX reception hardware.

Each pixel includes a standard RS-485 receiver and a small microcontroller.  The receiver chip converts a balanced differential signal into a standard logic level that the processor can understand.  Each pixel requires three consecutive DMX channels: one each for red, green and blue.  Red is always the first channel, green the second and blue the third.

Of course, the start address of each pixel can be set to any number between [1 510].  Thus, 170 pixels can be discretely controlled by a single DMX universe.

Point Source Pixel DMX Splitter / Driver
Pinout / Wiring Data for DMX Splitter / Driver
Ribbon Cable Adapter
Ribbon Cable Adapter

A driver board has headers for  power  and DMX in/through.  It contains an RS485 receiver chip and 2 RS485 transmitter chips.  Since RS485 is meant to be used in shielded, twisted pair cables (rather than ribbon cable) we decided it made most sense to drive moderately short (10′  – 20′ long) cables.  If the ribbon cable is much longer, signal integrity of the high-speed DMX data is compromised.

Thus, up to 32 driver boards may be daisy chained together.  Each driver board has two outputs.  Each output can drive between 1 and 32 RGB pixels.

The system’s backplane is based on regular 10-conductor ribbon cable.  Since these are very light cable gauges, we use 4 conductors each for power and ground.  The remaining pair transmits the DMX data.

To build the backplane, we installed 10 conductor IDC female connectors at regular points along the ribbon cable.  Spacing between 6″ and 8″ seems to work well.

Point Source Pixels have a 10-pin male header, which conveniently attaches to the ribbon cable backplane.  Naturally, the header pinout matches that of the splitter / driver boards.

Sample part numbers from www.jameco.com:

  • 10 pin male header #67821, $0.21 @ qty 10 (included on assembled circuit board)
  • 10 pin female IDC socket #32492, $0.25 @ qty 10
  • 10 conductor ribbon cable #643815, $15.18 / 100′

System Power

Each pixel has a built-in voltage regulator which converts Vin (usually 8-12V DC) to +5v required by the processor and LED.  Note that the RGB LED is driven from the processor’s pins.  This means that the board input voltage can vary somewhat, so long as it is high enough that the voltage regulator can function properly.  We’ve had spectacular success using switching power supplies designed for / removed from computers.  They output +12v DC at many, many amps, and for a very reasonable price.  Try www.weirdstuff.com if you don’t have anything lying around.  At full power, each pixel draws just over 60 mA at 5 volts.  A bit of Ohm’s law shows that

  • .06 A * 5V = .3W
  • 12V / .3W =  40 mA drawn from a +12v supply.

So choose a properly sized power supply for the job.

Wiring diagram / pinout for DMX Driven Point Source Pixel
Wiring Diagram for Point Source Pixel
10mm Point Source Pixel - Close View
10mm Point Source Pixel - Close View

DMX Addressing

Naturally, for most effective operation each pixel requires a unique DMX address.

Since each board is too small to contain a DIP switch or display + pushbuttons, there are two ways a pixels DMX start address can be set.

  1. In firmware at compile time.  Using a PIC programer and Microchip’s MPLAB, each pixel’s start address can be hard-coded into program memory.  This works well (we’ve done it thousands of times) but gets tedious after a while.  Also, a special programming harness is required to access the proper pins on the chip.
  2. With an external programmer.  We designed the firmware to listen for a specific sequence of data, beginning with a ‘non-zero’ start code.  This start code / data sequence would never occur in a regular lighting system.  Think of it as a secret knock on the pixel’s back door.  When the proper data and checksum is received, the start address is stored in the processor’s permanent memory.  Changing addresses is easy and takes only a few seconds and can be done without any special computer programs.  See this link for a description and video clip of the field programming system.

That’s a decent overview of the system.  Questions?  What have we missed?  Send an email to john AT response-box.com.

Field Programmable Source Code!

Finally!

It used to be that we’d write the pixel’s DMX address in firmware, then compile and program each PIC.  It worked well but got tedious.

Several thousand pixels later, we’ve got field-programmable source code up and running.

Short version: the PIC listens for an alternate (non-zero, dimmer data always is zero) start code in the DMX stream.  That start code is followed by a special packet of data which contains, among other things, the new start address plus a checksum.  The chances of this particular packet occurring naturally in your lighting rig are one the order of 1 in 2^80.  That’s a 1 followed by 24 zeros.  At the time of this writing, this number is slightly higher than the new US national debt.

‘Programming’ packets can be sent at any time.

The new address is, of course, stored in the processor’s permanent memory.

The address is also displayed by the pixel on power-up. The red LED flashes once (.2 S duration) for each ‘hundred’ in the pixel’s address or once (.6 S duration) if there are no hundreds.

Likewise for green / tens and blue / ones.

Channel 1 = long | long | short

Channel 12 = long | short | short short

Channel 304 = short short short | long | short short short short

etc.

So now, all pixels can be factory programmed with the same firmware.  This saves us a tremendous amount of time.

Firmware works for point source, ‘mini’ and ‘classic’ pixels and is totally backwards-compatible with anything we’ve ever shipped.  It will also work in 3-channel mode on the through-hole DIY pixels.  Haven’t had time to mess with the 5-channel version.

Contact us for a .hex file if you want to re-burn your own pixels.  Or send ’em back and we’ll be happy to re-flash them with this new code.  Programmers are $46 and will be available soon in the online store.

Watch it work in the clip below. Click the arrows in the bottom right corner of the video frame for a full-screen version.


Setting Pixel Addresses in the Field from Engineering Solutions Inc on Vimeo.”>

Boring technical bits:

A normal DMX packet looks something like this on a ‘scope:

BREAK 0 X X X X X X X

Where 0 is the start code, which is then followed by between 1 and 512 8-bit channel values.

Our pixel programming packets have 11 bytes and look like this:

BREAK P I X E L S HH LL CHECK 0xFF

‘P’ is the upper-case ASCII character having a hex value of 0x50. ‘I’ is 0x49, etc. HH is the high byte of the new address. LL is the low byte of the new address. CHECK is the 8-bit sum of the high and low address bytes, overflow ignored.

Programming packets which don’t precisely match this format are rejected.

The pixel firmware doesn’t currently error-check the new address, so values between 513 and 65535 are technically valid. They’ll just never light up in any production lighting rig. However, the programmer firmware is range limited to [1 510]. What good would it do to park a 3-channel pixel at 512?

Pixel Installations in the Field

Here’s a small (but growing) collection of projects which include our DMX pixels.

mrpackethead from New Zealand sends this photo and video clip.  Pictured are 160 of the ‘classic’ RGB Pixels, based on daisy-chaining cat5 network cable.



If you’ve used pixels in a creative or exciting way, we’d love to hear about it.  Send your pictures, links or video clips.

8 Way DMX Splitter

Haven’t had time to test it fully yet, but these boards arrived early this morning.

UPDATE:  These work wonderfully well.  I’m using two (one for each universe) in my display this year.

It’s an 8-way somewhat-isolated DMX splitter.  

DMX in & through on XLR-5 jacks. DMX in section is fully isolated from the rest of the world.

8 RS-485 drivers in parallel with pinheader (.1″) outputs. These outputs are *not* isolated from each other, but each does contain a discrete driver chip.  A proper opto-splitter design would have each output powered by it’s own DC power supply and isolated from its neighbors with an optocoupler.  To get an isolated DC power supply, use a small DC-DC converter per channel or a small AC transformer + regulator per channel.

Expensive and complicated, but very safe.

I skipped the added isolation because my rig was all being powered from a single power supply and all gear was connected to a single AC outlet.  Your mileage may vary, of course.

I’m using these to route my rooftop data lines, but ran about 20 extra boards in case anyone else was interested. $5 each, and I’ll send you the full schematic and parts list.  

8 Way DMX Splitter
8 Way DMX Splitter