Multi-Point Bus, Repeaters and 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/splitters that a signal may transverse.

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

The input of the repeater amplifier is the same as the input of any other DMX512-A device; the output is the same of that of a DMX512-A 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 something bad happens on one leg, it will not take down the signal on the other legs of the splitter, 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.

Mergers

DMX uses a serial bus topology for each cable segment in which multiple cable segments may be joined together using splitters (regenerative repeaters), but there is a maximum segment length. In any system, there is only ever one controller which generates the control signal. In some applications it is desired to have a backup controller or to combine the signals from two independent controllers. This may be done using a DMX merger, one device with two or more independently operating receivers, that accumulates copies of the DMX frame from each bus and uses a rule to combine these to form a single DMX frame.

A merger is implemented using a microcontroller-based (or PC-based) device with multiple receiving input streams and a combined output stream. The input streams may be on separate DMX busses (i.e. using line receivers) or could be sent using one cable - as is the case for transmission over Ethernet - where each Ethernet frame identifies the Universe to which it belongs. Merging devices hence contain a complete controller logic when sending the regenerated frame and hence can change the timing or even the cable technology - e.g. converting to and from Ethernet framing (e.g. by sending DMX frames using ACN or ArtNet framing).

A DMX merger accepting inputs from two DMX universes (at rear), and combining the signal to form a single universe, before splitting this to feed 6 cables.

In some applications it is desired to have a backup controller or to combine the signals from two independent controllers. This may be done using a DMX merger, one device with two or more independently operating receivers, that accumulates copies of the DMX frame from each bus and uses a rule to combine these to form a single DMX frame. Common rules are:

Some devices also allow translation in LTP or HTP modes, in which the slots values from one interface are shifted by an offset before merging. For example slots 1, 2, 3, etc may be mapped to 17, 18, 19, etc if the offset were set to 16. In this example it would allow two simple 16 channel controllers to be used together so that they can control the first 32 channels of the DMX bus.

DMX Merger accepting two DMX universes as input and then generating a DMX output for two cable segments.

Merging devices contain a complete controller logic when sending the regenerated frame and hence can change the timing or even the cable technology - e.g. converting to and from Ethernet framing (e.g. by sending DMX frames using ACN or ArtNet framing).

Example 1 of HTP Merging

This example illustrates a merger that combines one set of DMX slots on the first input continuously sent with values 00 FF 00 7F 00 and a second set of slots on a second input continuously sent with values 00 00 FF 04 01

There are two Input Universes 1, 2; and one Output Universe 3. Universe 3, is a HTP Merge of Universes 1 and 2:

The corresponding merged output (Universe 3);

Therefore the output on Universe 3 is: 00 FF FF 7F 01

Example 2 of HTP Merging

There are two Input Universes 1, 2; and one Output Universe 3. Universe 3, is a HTP Merge of Universes 1 and 2.

The data observed on the inputs is:

Slot Number: 0 1 2 3 4

Universe 1: 00 11 02 FF 11 ...
Universe 2: 00 00 02 00
12 ...

For slot 1: 11 is higher than 00, hence the output for slot 1 is 11

For slot 2: 02 is the same as 02 hence the output for slot 2 is 02

For slot 3: FF is higher than 00, hence the output for slot 3 is FF

For slot 4: 12 is higher than 11 hence the output for slot 4 is 12 (this value is updated when Universe 2 is received).

The output is:

Universe 3: 00 11 02 FF 12 ...


See also:

External links:


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