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DMX Merge is the ability to take two or more incoming signals and merge them together and output as 1 DMX signal.
This is useful if for example you have a Traditional DMX Lighting Console and a DMX USB Interface that both only have a DMX out signal. So you could be controlling the show from the computer and overide certain channels from the console. For example you have a show running and you realise that the spots are not bright enough or the wrong colour, so you could move the corresponding faders on the traditional lighting console to a higher position to achieve the desired effect.
Differing DMX Mergers have different modes of operation, such as HTP or LoTP on the in of Line A, others it might be LTP on the in of Line A. Another mode is merge where as long as the signal is not changing on either line the the signal comes from Line A, then which ever line changes then that line goes into LTP mode.
Please check with the manufacuters before buying a DMX Merger as it might not do exactly what you require or expect it to do.
Both the examples below are rack mounted units and have differing modes of operation.
The reverse of a DMX Merger is a DMX Splitter.