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DMX splitting


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Hello Blue room

 

I'd just like to clarify something.

 

On most decent intelligent lights they have both 3 pin and 5 pin DMX connections. I read somewhere that you should only use 1 input and 1 output, whereas I saw recently one input being used, and then both outputs being used. This therefore used the moving light as a DMX splitter, and the daisy chains all seemed to work fine. I just wondered whether anyone knew the correct answer to this, or whether actually it really doesn't matter.

 

Thoughts would be appreciated as I've got a show coming up where the option to split the DMX would save a lot of cable!

Edward

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in all the units that I have had a close look at, the 3 and 5 pin connectors are simply in parallel. as such, using both outs would give you a parallel split without any isolation between the 2 legs.

 

Whilst such parallel splits may work in practice, they will frequently give problems, particularly if the 2 split legs are off significantly different length.

 

Given the availability and cheapness of isolated DMX splitters, I personally would not carry the risk.

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I've done it plenty of times on smaller rigs where I 'dodge' out of the second output to go to maybe one or two lights. Useful if you want to save turnarounds between fixtures where your using 5 pin cable and only one light is on 3 pin! I certainly wouldn't do it on a larger rig.

 

Like CSG has said, buffers are cheap to buy or hire so thats going to be the safest way.

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Ok thanks guys,

 

As this is a small show with a small rig I'll risk it this once! I'll bear this in mind for future events though.

 

Many thanks

Edward

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This article by Texas Instruments The RS485 Design Guide discusses stub lengths on page 3 and gives maximum stub lengths for various driver rise times. Being a pessimist for the design phase and an optimist for the useage phase, you should only allow a maximum stub length of 7 feet. (American company). So it appears that you may get away with splitting out of a mover up to a maximum stub length of 38 feet, but it is not good design practice, unless you know that the mover has separate RS485 drivers on the 3 pin and 5 pin outputs. I would certainly advise using terminating resistors on the stubs.
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I do know someone who took DMX to a stand then split it to four lanterns with a fan of twin flex, and it worked. BUT it shouldn't have done, and DMX done off standard may bite back. Keeping to the spec. is most of making sure that a fully variable system will continue to work as intended.
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I've done it once and paid the price. For two thirds of the gig it was fine, but then started to get random flashes on the fixture I'd broken out of. Both of the end fixtures were terminated. I had to get the SM to pull the power on the affected lights and complete the rest of the night without them.
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Remember that DMX lighting is called "intelligent" because it KNOWS when the sponsor is in the house and there is live broadcast and it falls down just then if it's not wired right.

 

There is a LOT to be said for Active splitter boxes. One DMX line in and about four separate but identical lines out. This solves any issues with having too many units on the line, eases wiring along several lighting bars, and isolates odd fittings that don't play nicely with all the others. (I used to have a set of colour changers that would only work in one order which is all wrong to the spec.)

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