20.4.2.5 DMX

Digital Multiplex 512 (DMX) is a protocol used typically for stage lighting and effects. It supports a ‘universe’ of up to 512 channels and is typically implemented using EIA-485 at the physical layer. DMX communication is one way only, with the controller only sending messages and the client device only receiving. There is no error checking or confirmation that a command has been received. DMX operates at a baud rate of 250k with no parity and two Stop bits. A DMX message frame consists of a header and up to 512 ‘slots’ (data bytes). A client device can be configured to accept more than one slot, given start and stop assignment values.

A DMX message frame consists of a Break, a Mark After Break (MAB), a start code and finally, the slot data bytes. The MAB is three bit times in length. The start code specifies the type of data, and it is typically at 0x00. Figure 20-19 shows a DMX frame.