DMX distribution idea

[Aquarius]

So, some background about our theatre:

We have 2 DMX ports in the whole thing. 1 in the booth, which the light board (ETC Express 24/48) is always plugged in to. The other one is on the stage right house light control panel.

The problem:

We have a dance concert coming up, and the choreographer requested fog machines. We recently converted our fog machines to DMX, as we got tired of running them manually and wanted to run them straight from the board. We normally position our fog machines just upstage of the proscenium arch, and have the dressers run them from there. However, now that we converted them to DMX, we would need to either run an XLR line over the arch (way too much time and cable) or over the dance floor (ugly, and a tripping hazard even if it was taped down).

The solution:

The plus side to our theatre is that we have mic ports coming out the wazoo. Each of the foggers is conveniently located right next to a pair of them, and we don't use any mics for dance shows. We came up with this solution: run DMX line from our 1 DMX port SR into the SR fogger. The cable from the XLR out port on the SR fogger runs into one of the mic ports. The input line on the SL fogger gets plugged into another mic port. Up in the booth, we have a patch bay. Using stereo patch cable, we patch together the mic ports that both of the foggers are plugged into.

Voila! We now have a way of extending DMX to anywhere in the theatre that we have a pair of mic ports (which is basically everywhere). And it even works well, despite the length of (hidden) cable involved.

Anyone ever tried this before?

[Thefoxygranpa]

Has anyone done it before? Yep, many times. When searching older topics this question comes up a lot and in short, it will work, sort of. I'm running older trackspots down an audio snake line from the booth in back of house to a jack on stage, and it has worked just fine for the past couple of months.

The install wire may not be DMX spec and therefore can create some issues, as well as your patch cords and the wire used on the patchbay themselves may not be on spec with DMX. And as always, you should terminate your signal if you are not already

Hope its working well for you!

[Tex]

Originally Posted by Aquarius

Voila! We now have a way of extending DMX to anywhere in the theatre that we have a pair of mic ports (which is basically everywhere). And it even works well, despite the length of (hidden) cable involved.

Anyone ever tried this before?

I'm assuming that your patch panel is comprised of XLR connectors? Mine is 1/4" stereo phone plugs that do not allow DMX to pass through.

[mstaylor]

It will probbly work but don't be surprised if weird things happen. Standard mic cable isn't made to handle DMX. If you are connecting the two sends in the booth and are sending the signal back to the stage then to the DMX imput on stage then back to the booth, that's a lot of cable length and can create it's own problem.
A better solution may be to run to the pit floor, across the arch and back to the next fogger. The other option is to place your foggers upstage and cross against the upstage wall. Personally I would go over the arch but that's me.

[derekleffew]

Originally Posted by Aquarius

...Anyone ever tried this before?

Bad idea. Thefoxygranpa has noted the issue of using audio cable to carry a DMX signal, so I'll address another problem.

The system is designed to allow for a console to be placed either in the booth or backstage right. The wiring probably goes from the booth to the SR input, where it is "Tee'd," then on to the dimmers. Using the stage input as an output creates a "Y," which is not permitted per the DMX512/EIA-485 standards.

Yes, I've done it, but only for non-critical applications (smoke machines probably qualify) and always/only with an opto-splitter immediately after the onstage input. Using an input as an output also requires a FF turnaround, which is usually an indication that you're doing something at least unintended, if not outright wrong.

[epimetheus]

Originally Posted by Tex

I'm assuming that your patch panel is comprised of XLR connectors? Mine is 1/4" stereo phone plugs that do not allow DMX to pass through.

What kind of 1/4" do you have that doesn't allow DMX to pass through? I agree that the audio cable isn't DMX spec and that there is probably a 50/50 chance of this working, but a 1/4" TRS has all the same connections as an XLR, unless your patchbay has common shields or something like that.

[fredthe]

Originally Posted by derekleffew

The wiring probably goes from the booth to the SR input, where it is "Tee'd," then on to the dimmers. Using the stage input as an output creates a "Y," which is not permitted per the DMX512/EIA-485 standards.

This actually brings up another issue... termination. IF there are two points feeding the dimmers (booth and house), then presumably the dimmers are terminating the connection. Which means that you can't terminate the run to the foggers, else you'll be double terminating.

Yes, it might work. No, it's not the right way. (See the thread on "Jerry Rigged")

-Fred

[JD]

Be aware that most "pulled" audio cable (what's in the conduit) is 64 ohm. (Often Belden 8761) DMX will work as mic cable, but mic cable should not be used for DMX. Pulling DMX would be a good idea as you can splice new to old and then use the old cable to pull the new one through. (be careful!) Whenever splitting the run, use an active opto-splitter. Each output line should then be terminated. As for using stereo phone jacks for DMX, sounds like a good way to static spike the line and damage something. With XLR cables, pin #1 is designed to make contact first, which drains off any static charge. Phone Jacks hit tip first, so any stray voltage will spike the actual data line.

[epimetheus]

Originally Posted by JD

Be aware that most "pulled" audio cable (what's in the conduit) is 64 ohm. (Often Belden 8761) DMX will work as mic cable, but mic cable should not be used for DMX. Pulling DMX would be a good idea as you can splice new to old and then use the old cable to pull the new one through. (be careful!) Whenever splitting the run, use an active opto-splitter. Each output line should then be terminated. As for using stereo phone jacks for DMX, sounds like a good way to static spike the line and damage something. With XLR cables, pin #1 is designed to make contact first, which drains off any static charge. Phone Jacks hit tip first, so any stray voltage will spike the actual data line.

Good point about the tip making contact first. I had thought of that. If you were to try to run DMX through a 1/4" patchbay, I certainly wouldn't try to make a connection with the system powered up.

[BillESC]

Just a quick note about running digital signals through installed house audio cables.

We were contracted to provide lighting and effects for a dance recital in the brand new 4 plus million dollar auditorium at West Point High School, West Point, VA.

To make things easy I brought a half dozen NSI 4 channel dimmer packs to plug the effects into and plugged the microplex into an audio channel behind the procenium wall. As soon as I powered up the packs, all kinds of things started to blink, wink, strobe, etc.

After trouble shooting the situation, I discovered all the the house audio were run with just two conductors. Nice job audio contractor.