The DMX Start Code

The Start Code value is sent as first slot of each frame. Normally this is set to zero to indicate a frame carries control data. 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

Table: Some Common Start Code Specified for DMX

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 0x17 (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:

• Slot 1: Page number of one of a possible 256 text pages.
• Slot 2: Number of characters per Line. Indicates the number of characters per line that the transmitting device has used for formatting the text. A slot value of zero indicates the receiver should ignore this field.
• Slots 3-512: Consecutive display characters in ASCII format. All characters are allowed and where a DMX512 text viewer is capable, it shall display the data using the ISO/IEC 646 standard character set. A slot value of zero ends the ASCII string. Any remaining slots transmitted after this zero slot are ignored.

System Information Packet

Manufacturers of control consoles are encouraged to transmit SIPs, either as a background to normal processing or, in conjunction with the special test packet, as part of their suite of system test functions. The SIP contains information about the transmitter such as its software revision, but also provides information about the data being sent. The alternate START Code 0xCF (207 decimal) is reserved to identify a SIP.

A SIP Packet carries 24 data slots., which includes a checksum.

One of the current problems with testing of DMX512 installations is that it must be done with static test packets – certain modes of testers cannot be used while a console is actually controlling equipment,. The interleaving of SIP’s with control data frames allows some degree of live testing, that can be monitored by a receiving device.

One slot in the SIP indicates the (originating) DMX512 universe currently transmitted on this link. 00h is not used. Valid values 01h - FFh (1 decimal - 255 decimal).

Reception of as SIP can also reveal other equipment that might modify or repeat a DMX frame as it travels along the control bus. Processing devices and media converters that process SIPs shall be required to insert their own Manufacturerís ID into the SIP packet. This allows for a packet processing history to be traced back though a complex installation of products.

For systems requiring a more reliable transmission, a sender has the option of following every normal frame with a SIP packet. This provides an integrity check and loss detection, although this would degrade data throughput. It could also be used with systems that send packets of fewer than 512 DMX512 data slots or refresh data at less than the maximum rate.

See also:

• DMX Frame
• http://tsp.plasa.org/tsp/working_groups/CP/DMXAlternateCodes.php.

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