Multi-Point Bu and Repeaters (DMX Splitters)

The maximum number of receivers that may be placed across the control bus is given by the receiver input resistance. In DMX-512 the input impedance of 12 k Ohms restricts this to 32 devices on a single bus. (32 parallel receivers have an overall impedance of 376 Ohms) This is the safe value for cable runs up to 300m.

DMX operates at a data rate of 250 k baud. Reducing the number of receivers can allow much longer lengths of cable at the same baud rate. For instance, a cable run of up to 1000m may be possible with some line drivers, with only two devices. Many practical products can not manage to drive a cable of even 300m!

Repeaters and Splitters

When very long cable segments are employed (e.g., segments greater than 300m), or more than 32 devices are required, it is necessary to boost the signal through a regenerative repeater amplifier, also known as a buffer amplifier or regenerative repeater. The key to digital transmission is not the only to use amplifiers, but to use amplifiers as a part of regenerative digital repeaters. A repeater amplifies and equalises the received signal, and then digitally samples the signal to decode the bits being sent (i.e. recognises each bit in turn). Once decoded, the digital bits may be sent a fresh down the cable to the next repeater, regenerating a signal at the output of the repeater which is as good as the original sent signal. By using repeaters at suitable points along the cable (before the signal deteriorates below the level at which it may be reliably decoded), arbitrary long distances may be reached with little probability of bit errors.

Splitter amplifiers are like repeaters, but have more than one output. They can be small converters, rail or rack-mounted units linking cable to cables. They preserve and duplicate digital signals, effectively expanding DMX control bus (limited to 32-devices) to any number of connected devices.

RDM requires repeaters to support half-duplex, in which the direction of the ports may be changed as the RDM protocol is used. This requires an RDM repeaters to interpret the control signals sent on its uplink and to understand when downstream devices are expected to transmit (including implementing timers to recover from loss of a control frame.). This requires a micro-controller based design. The timing requirements of RDM also imposes a limit on the the maximum number of sequential repeaters that a signal may transverse.

A DMX controller for 1 universe driving a DMX bus that feeds a repeater to control two cable segments receiving the same DMX frames.

The input of the repeater is the same as the input of any other DMX device; the output is the same of that of a DMX transmitter. The repeater receives the signal from the input cable segment, converts to logic level and then outputs a "clean" copy of the signal to the output cable segment. This allows connecting areas that are more distant and/or are not linear.

The outputs may also be optically isolated. Optical isolation is an electrical barrier between the input to the output and between the other outputs. If theer is a fault on one output, it will not take down the signal on the other putputs of the repeater, or go back up the line and damage your really expensive controller. DMX is sensitive to grounding issues, so using an opto-isolator will help solve those problems.

Each DMX cable segment carries only one set of DMX channels, known as a "Universe". Multiple universes can exist where more than 512 channels are controlled, however they must be cabled individually. This is usually common practice where one bus controls lighting whilst another controls moving objects. A system information packet (SIP) can be used to provide information to receivers. The SIP packet contains data on the current universe, number of packets sent and received since last SIP.

In larger systems employing several DMX universes it is now common to send the console data via Ethernet. A set of up to 512 data slots identified by universe number. In ArtNet and ACN framing, each packet contains a universe number identifying the universe it carries.

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External links:

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland (2016)