Two protocols, different wiring
DMX512 is an RS-485 bus: a controller sends channel values to addressed fixtures over a twisted pair, with up to 512 channels per universe. SPI pixel strip is driven differently: the controller clocks data through the strip in sequence, so the position of each pixel in the chain defines its address.
That difference decides the wiring layout. DMX can branch and be addressed individually; SPI has to follow the data direction and usually runs in a continuous chain per output.
Counting universes and pixels
One RGB pixel needs three channels, so a universe of 512 channels covers 170 pixels. An RGBW pixel needs four channels, which reduces the count to 128 per universe. Pixel grouping matters too: some strip drives one IC for three or six LEDs, so a 'pixel' in the protocol may not be a single LED on the board.
Agree the grouping with the supplier before calculating controller capacity. A 60 pixel per metre strip that groups six LEDs per IC behaves like 10 addressable pixels per metre in the control system.
Power injection and common ground
Pixel strip draws current at every pixel, so long chains suffer the same voltage drop as single-colour strip, with the added risk of data corruption when the supply sags. Inject power every few metres and keep the data line's 0 V reference common with the supply.
For exterior runs, protect the data and clock lines and keep them away from mains cables. A short data stub at each injection point reduces reflection problems.
- Common 0 V between controller, driver and strip.
- Power injection at intervals, with the 0 V joined at each point.
- Termination or a series resistor where the controller expects it.
- Keep data cable runs short and avoid parallel routing with mains.
Commissioning order
- Test one short chain on the bench before installing the full run.
- Confirm pixel order and direction in the controller before addressing.
- Set the output brightness limit so the supply is not loaded to its maximum.
- Record the addressing map, because it is the document the next technician needs.
When to use which
Use DMX when the project has a lighting console, multiple fixture types or branch distribution. Use SPI when the effect is a continuous media surface driven by a single controller output, and the chain layout can follow the data direction.
Key takeaways
- DMX is addressed over a bus; SPI clocks data through a chain.
- One RGB universe covers 170 pixels; RGBW covers 128.
- Common ground and regular power injection prevent most flicker.
