cdcarter Posted November 1, 2005 Share Posted November 1, 2005 My school is adding two 4 channel dimmer "Porta-Pak" style dimmers. They are a non-standard 3-pin DMX model, so we went out and bought an adapted to make it a 5-pin. We also realized that the DMX in-house is also male, so we are in the process of getting a coupler. As this is happening, I was getting background information about our system. The people who wired our theatre were not experienced in DMX, and wired it as a telephone style network (central hub), not as a daisy chain DMX network. This causes occasional feedback down the wire, but very little. Because the porta paks will be right next to the DMX hub, I have been told that it does not need to be terminated, but I am thinking this may cause feedback, due to the way the system is wired. I was wondering if anyone knew if adding the dimmers, with or without the terminator plug, would cause an upset in the balence in our system that is keeping the feedback levels down. Link to comment Share on other sites More sharing options...
dbuckley Posted November 1, 2005 Share Posted November 1, 2005 Oh dear. I see two possible approaches to tidying your system up so it works properly. Firstly, when the wired the place for DMX, what cable did they use? If you are really liucky they will have used two pair (four core) DMX cable, or possible 4 pair Cat5, in which case you are in luck. You can use each cable as a "there and back" from your star, using both pairs of cable. In this case you need one terminator right at the end of your newly built line. Or, if they used just one pair cable (or mic cablke) then you are a bit stuffed. You need a splitter at the star point, see this current thread , and each run needs to be terminated. Running the thing as a straight star wont work at all well if there are any significant lengths involved. If you elect to leave your system in its current unreliable state then what you should do is loop your new dimmers in right at the start of the system, without terminators. Link to comment Share on other sites More sharing options...
themadhippy Posted November 1, 2005 Share Posted November 1, 2005 easiest thing is to unplug the input to the DMX hub,and plug that into your first dimmer,daisy chain the dimmers together if youve nothing upstream terminate at he last dimmer and leave the hub and rest of wiring disconected,othewise plug the hub into the output of the last dimmer and pray/hope for the best Link to comment Share on other sites More sharing options...
back_ache Posted November 2, 2005 Share Posted November 2, 2005 The people who wired our theatre were not experienced in DMX, and wired it as a telephone style network (central hub), not as a daisy chain DMX network. The IT industry moved from daisy-chain networks to star over a decade ago, and in a DMX situation would allow for better diagnostics and reliablity This causes occasional feedback down the wire, but very little. That would be more likely be if the end of each chain coming from the hub isn't terminated and if the "hub" was just wired the cables together rather than spilting and buffering Because the porta paks will be right next to the DMX hub, I have been told that it does not need to be terminated The end of a DMX chain should alway be terminated, so in the case of star wiring at the end of chain coming of the hub. Link to comment Share on other sites More sharing options...
cdcarter Posted November 2, 2005 Author Share Posted November 2, 2005 Our DMX "hub" is in our dimmer rack. The rack is the circuit breaker, the dimmers, and a huge jumble of cables spliced together that run to the dimmers, and three DMX male ports in the house. We do not have the ability to rewire without using the electricians union, which is not cheap, and our budget is pretty low, and we need to save it for gels. That would be more likely be if the end of each chain coming from the hub isn't terminated and if the "hub" was just wired the cables together rather than spilting and buffering They just wired the cable together. Every end is terminated as of now, and we are looking for money to buy a terminator plug for the new dimmers. Link to comment Share on other sites More sharing options...
paulears Posted November 2, 2005 Share Posted November 2, 2005 Electronics is not covered by the electricians union - even if there was such a thing. DMX is data, so is more likely to be the responsibility of the IT department, but if it is a new installation, then it hasn't been done properly - and should be put right. It may function now, with only a few things attached to it, but you can bet it will at some time fall over. Maybe it would be good if the faults start to happen early on, while there is some chance of getting it fixed? A terminator on each leg may well mess up the impedance completely - the dc resistance will go down much further than it should, and that isn't a good way forward. Link to comment Share on other sites More sharing options...
tadawson Posted November 3, 2005 Share Posted November 3, 2005 Electronics is not covered by the electricians union - even if there was such a thing. DMX is data, so is more likely to be the responsibility of the IT department, but if it is a new installation, then it hasn't been done properly - and should be put right. It may function now, with only a few things attached to it, but you can bet it will at some time fall over. Maybe it would be good if the faults start to happen early on, while there is some chance of getting it fixed? A terminator on each leg may well mess up the impedance completely - the dc resistance will go down much further than it should, and that isn't a good way forward. Not to sound too cynical, but this is the kind of bogus work that I would expect if a Union electrician did the initial work - it is really out of their area of skills . . . . . . At this point, the best you can do without total rewiring, is to take ONE leg off the big, ugly, splice, and feed it to the first dimmer pack. Then take temporary cable, and daisy chain the rest, and terminate the last. If you are in a star like that with more than one terminator, you have an unacceptably high load on the console DMX source, which may cause flaky behaviour in downstream devices, or outright failure of the console. - Tim Link to comment Share on other sites More sharing options...
Jivemaster Posted November 3, 2005 Share Posted November 3, 2005 There ars two obvious choices, after fully determining the existing system, fit a DMX isolated splitter, or re-fix the wires to make a daisy-chain. Look at the desk, mine has four separate chanels out via plugs so four wires out is -though radial looking- quite OK as the desk has four DMX drivers -- does yours? Link to comment Share on other sites More sharing options...
paulears Posted November 3, 2005 Share Posted November 3, 2005 There are quite a few rack mount DMX splitter/hub devices available, why not just get one? Link to comment Share on other sites More sharing options...
back_ache Posted November 3, 2005 Share Posted November 3, 2005 There are quite a few rack mount DMX splitter/hub devices available, why not just get one? I agree that has to be the way, the current "hub" sound like a car crash waiting to happen and there is no solution that does not involve money. Link to comment Share on other sites More sharing options...
cdcarter Posted November 3, 2005 Author Share Posted November 3, 2005 Once our current show closes and we add the new dimmer packs, I am going to try and isolate one leg of the star, and put all the dimmers on it. The console will be on a seperate leg of the star, like it currently is. After consulting with some others, I have been told that this move will remove feedback consideribly, and should allow for the new dimmers. Getting a splitter/hub device is out of the question unless I can get it for less than some bits of DMX cable. I would love to add a splitter or just re-wire the goddamn thing, but I highly doubt that will happen soon. The only reason we are adding these dimmers is that we got them for free (they were meant for the new blackbox theatre, but they scrapped that idea, so they went into the main house) and that the new LD likes to add many many more instruments than we have dimmers. This also means a very creative patch (24 channel board). Link to comment Share on other sites More sharing options...
krisn Posted November 3, 2005 Share Posted November 3, 2005 Our DMX "hub" is in our dimmer rack. The rack is the circuit breaker, the dimmers, and a huge jumble of cables spliced together that run to the dimmers, and three DMX male ports in the house.I hate to ask a stupid question, but are you sure your not describing a cans circuit, instead of DMX? Cans can be run in a star formation from a central hub or daisy chained without a problem. Furthermore you say you have three sockets in the house. This is a little unusual for DMX in/outlets but would make sense for a comms installation (then again I don't the site). If they are DMX outs then I would be surprised to find them all over the place, normally I would expect DMX outlets near power connections or overhead for intelligents etc. If they are DMX ins then I would expect them to be in sensible positions e.g. plotting location or by the stage. As you said they are Male ports, then this would normally indicate they are DMX inputs and so should not be used for plugging into dimmers. If working as inputs it would be more justified using them in a 'star' configuration, assuming they had 3 inputs and then got sent off on one output route. Link to comment Share on other sites More sharing options...
cdcarter Posted November 4, 2005 Author Share Posted November 4, 2005 It is a DMX system, it is controlled by a DMX board. One of the upshots of the way the system is wired is that inputs and outputs are the same. We have actually two runs to the house, and one in the lighting loft. Link to comment Share on other sites More sharing options...
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