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Combining the ground of two DMX universes in one cable?


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The problem is we only have one data cable coming from FOH to back stage and we really need to run two universes (from different desks) but the cable only has 4 cores and ground, so just wondered people opinions on using two cores for each +&- then combining the ground conductors to the one? Running another cable is not possible.

Anybody's thoughts much appreciated Thanks.

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I've done exactly the same in an identical situation and it worked fine. It's even approved by the DMX512 spec. :) You can get some confused looks as the break out cable looks identical to a passive Y from the outside. The following is from the BR Wiki.


This is the pinout as defined in the standard for 5-pin connectors (pins 4 and 5 are also mentioned in the standard, pin 4 for an extra data -ve, pin 5 for a data +ve, but are seldom used.
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Surely that's no different to the possibility of there being a slight difference in ground potential anywhere else in the DMX chain.

The reason it's different is because the Console (DMX source) is defined in the spec as the only place where DMX Common may connect to Ground.


All your fixtures (should) have an effectively 'floating' DMX Common, thus various ground potentials don't matter.

(You do find fixtures that don't, and they can cause all kinds of difficult-to-diagnose faults.)


If however you had two consoles and connected their DMX commons together, they would (usually) both be connected to the real Earth so any difference in their ground potentials would cause a current to flow between the two consoles through that DMX common wire. This would be bad.


If both are coming from the same console (or other DMX source), that's clearly not going to be an issue.

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