BECKHOFF DK9222-0311-0029 - Manual (Page 2)

I/O
Light
Application Note DK9222-0311-0029
Fig. 2 Accent lighting with DMX
Characteristics of the bus system
Transmission according to RS485
DMX uses serial RS485 physics for data transmission, in which the information (DMX frame) is transmitted as a difference
signal on two lines. RS485 is relatively immune to EMC interference. A DMX frame encompasses a maximum of 512 bytes,
wherein a byte is designated as a channel, slot or value. In order to maintain high performance of the system despite serial
transmission, the controller sends a frame with 250 kbaud to all devices (Broadcast). Each device is assigned a certain address
range during configuration of the DMX strand, therefore, a device only accepts that part of the data from the frame that
is placed in the address range assigned to it. Due to the RS485 physics, a maximum of 32 slaves are allowed in one strand
without repeaters.
Utilization of the frame
The frame of the DMX protocol, which is up to 512 bytes long, is termed a ‘universe’ and one byte within the universe is termed
a ‘channel.’ This channel represents a certain device setting in 256 increments (8-bit), e.g. dimming, color, focus etc.
A universe thus encompasses up to 512 channels, which would theoretically correspond to 512 DMX slaves. However, RS485
is physically limited to 32 slaves and moving light sources take up additional channels for further parameters such as tilt, pan
and speed (in 8 or 16-bit resolution). If many moving DMX slave devices (high number of channels due to many features) are
connected to a DMX master or a universe, then 512 channels may not be sufficient for the maximum number of 32 slaves
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For application notes see disclaimer on the last page