BECKHOFF DK9222-0311-0029 User Manual

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Application Note DK9222-0311-0029

Light

Fast communication with the DMX protocol

Keywords

EL6851
DMX
DMX512A
RS485
Show technology
Lighting
LED
Lighting control
Lighting technology
Master
Slave
XLR
Light
Stage
Show

This application example describes DMX – a standard transmission protocol for lighting technology

– and the Beckhoff DMX EtherCAT terminals EL6581 (- 0000: master, -0010: slave), with which the PC- and

EtherCAT-based control technology can be used for show, stage, special effects and drama applications.

DMX

DMX is the standard protocol for controlling professional stage and lighting effects equipment, which is used, for example, in

the dynamic lighting of showrooms and salesrooms as well as for exclusive displays of light and color in high-profile buildings,

such as hotels and event centers. As the bus system used in professional lighting technology, DMX works with RS485 physics.

DMX stands for ‘Digital Multiplexing’ and enables serial data transmission with a data rate of 250 kbit/s. Color mix and

brightness values are transmitted to DMX slaves in the form of static light sources (e.g. spotlights). In the case of moving

light sources (e.g. moving heads and scanners), angles for pan/tilt and motion profiles stored in the device are additionally

communicated. The topology configuration corresponds to daisy chain cabling (all slaves in a ‘universe’ in series); therefore, the

slaves have an input and an output port. Three-pole XLR plugs are used for cabling.

Dimmer Color changer Moving Head

Scanner LED

color changer

Fig. 1 Typical DMX slave devices

For application notes see disclaimer on the last page

Stroboscope

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Application Note DK9222-0311-0029

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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

For application notes see disclaimer on the last page

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Application Note DK9222-0311-0029

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under certain circumstances. In such applications, several DMX universes must be connected in parallel.

Data transfer rate

DMX requires approx. 22 ms for the transmission of the data if the full frame length is transmitted, which is equivalent

to a refresh rate of 44 Hz. The human eye would perceive changes of light at this low frequency as ‘non-harmonic’ and

‘discontinuous.’ Fade up/down transitions only appear to be even at frequencies above 200 Hz. The data transfer rate needed

for harmonious fades/changes of light can only be achieved by reducing the amount of user data per universe and as a result,

the transmission duration of the frame is reduced.

Ratio of data transfer rate to user data quantity

No. of channels Refresh rate

512 ~44 Hz
64 ~300 Hz
16 ~1000 Hz

Note:
not all DMX Slave devices can work with a variable high refresh rate,
since the internal data processing is sometimes too slow.

Fig. 3 Increasing the DMX refresh rates

For application notes see disclaimer on the last page

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