The DMX Start Code

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

Non-zero values indicate alternate frame formats. For example, the value 0xCC (binary 110011000) is used to indicate a frame sent using the Remote Device Management (RDM) protocol. 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

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
1100 1111 System Information Packet Identifies a DMX Universe
1111 1111 Dimmer Curve Select

The number of Alternate START Codes is limited to 255 possible values and many of these are already assigned or in use. Therefore, it is necessary to conserve the number of ASCs that remain for those few occasions when only an ASC will serve the purpose. A full set of Start Codes is specified by the Control Protocols Working Group PLASA Technical Standards Programme.

ASCII Text Packet

The Alternate Start Code 17h (23 decimal) designates a special packet of between 3 and 512 data slots. The ASCII text packet allows equipment to send diagnostic information coded per the American Standard Code for Information Interchange and formatted for display.

Slot allocation is as follows:

See also:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2014)