The DMX Start Code

The DMX frame starts with a Mark After Break (MAB) of at least 12 µS. This allows a receiver to reliably detect the first slot.

The Start Code valus is sent as first slot of each frame. Normally this is set to zero to indicate it is dimmer conrol frame. Simple devices ignore any start code larger than zero.

In this figure the green trace shows the signal from pin 3 and the blue trace shows the signal from pin 2. This illustrates the different voltages on a differential bus. In this figure It is possible to see the main aspects of a DMX Frame in this image both sides of the differential bus are seen. The MTBF (Mark Time Between Frames), in this case the controller set the MTBF to 2mS. It is also possible to see a break of 88uS and the MAB, mark after break. You can also see the first slot is a start code of 0 which tells any DMX device to expect a dimmer or light code. The signal then floats high as the microcontroller processes the next frame. The next transmission is the MTBS (Mark Time Between Slots) which pulls the signal to ground then transmits the first channel packet. The first channel contains a code of 255 in hexadecimal and so is seen as a transmission of binary 1ís. It contains a start bit and two stop bits (B, B2 and B3). These mark the start of the packet and the end of the packet.

The value 0xCC is used to indicate a command, status report, etc in Remote Device Management (RDM). By using this non-zero start code, RDM packets can be safely inserted between DMX data packets without older non-RDM aware devices attempting to read them

Start Code Values

Code Meaning Notes
0000 0000 Lighting Data Default format
0101 0101 Network Test All slots carry the same value
0001 0111 Text Packet Simple text message
1100 1100 Remote Device Management RDM Control/Response message
1111 1111 Dimmer Curve Select

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland.