Multi-Drop busses

Mergers

DMX uses a bus with a maximum cables segment length. Multiple cable segments may be joined together using repeaters. There is only ever one controller which tranmits the control signal.

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:

Highest Takes Precedence - in which the highest value in each respective slot is chosen.
Latest Takes Precedence - in which the last change of value in each respective slot is chosen. LTP is also called “override” in the channel groups as the output of this behaviour will override whatever the channel is currently set to and apply the new value.
Backup - in which the entire frame is copied from the first active interface.

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);

For slot 0: The output is 00 (both inputs are 00).
For slot 1: FF is higher than 00, hence the output for slot 1 is FF
For slot 2: FF is higher than 00 hence the output for slot 2 is FF
For slot 3: 7F is higher than 04, hence the output for slot 3 is 7F
For slot 4: 02 is higher than 00, hence the output for slot 3 is 01

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 ...