Roderick Posted August 30, 2018 Share Posted August 30, 2018 Today I was helping a friend make sense of a very, very poorly maintained venue where he just accepted a job as Technical Manager.One thing that had me scratching my head was this scenario:There are 5-pin DMX runs to all the permanent lighting bars. All are fed from a DMX 8-way distro in the control room.When I hook a LED Par directly to the desk, everything is smooth. If I run the same though the distro all is smooth.But when I patch it in the DMX outlet at the bar the intensity channel starts flickering.I can control it and turn it off, but as soon as the channel is brought up it starts flickering.I have tried 3 different units, same same. Nobody knows if it ever worked, any suggestions what to look for? Link to comment Share on other sites More sharing options...
david.elsbury Posted August 30, 2018 Share Posted August 30, 2018 Is each run terminated?I’ve seen installs where the screen is on pin 2 and DMX cold is on pin 1. I’ve seen them where cat5 was used and instead of using a twisted pair for the pins 2/3 they used solid colours e.g Brown&BrownWhite for pin 2 and Orange&OrangeWhite for pin 3. I’ve seen installs where pin 3 is dropped. I’ve seen DMX runs out of mains flex...How sure are you the install is done right? Link to comment Share on other sites More sharing options...
gyro_gearloose Posted August 30, 2018 Share Posted August 30, 2018 Since the fixture works when it’s connected to the console or the splitter, but not when connected to the bar outlet I’d say the problem is with the cable between the splitter and the bar. Use a known good cable plugged into the bar outlet that will reach back to the splitter, and meter the cable from the splitter to the bar. Also make sure nothing is grounding to the plug or socket shells. Link to comment Share on other sites More sharing options...
alistermorton Posted August 30, 2018 Share Posted August 30, 2018 Floating/broken ground or cold between the distro and the bar outlet? Or a connection between ground and the shell? Have you (or rather your friend) belled it through to make sure the connections are both correct and continuous? Link to comment Share on other sites More sharing options...
DrV Posted August 30, 2018 Share Posted August 30, 2018 Is each run terminated?Don't forget this also includes the run from the desk to the splitter. Many people consider (with good reason) that termination is unnecessary on really short runs but it does help to highlight situations where one leg of the run is broken. The use of an untwisted pair is a nasty one as you can only really see it by visual inspection of the ends - it will bell out correctly with a simple continuity test. Link to comment Share on other sites More sharing options...
eamon Posted August 30, 2018 Share Posted August 30, 2018 Might be worth checking that RDM is turned off on any of the DMX buffers et al...? Depending on the make of the LED par, the cheaper units do not like RDM. Link to comment Share on other sites More sharing options...
Don Allen Posted August 30, 2018 Share Posted August 30, 2018 I use a Network Cable tester with three pin and five pin XLR adaptors to test DMX install wiring. It will not show if they have used the wrong cat5 pairs, but will show basic wiring faults and you can do the testing by yourself as you have the transmitter at one end of the patch and the receiver at the remote end. Link to comment Share on other sites More sharing options...
timmeh2 Posted August 31, 2018 Share Posted August 31, 2018 Might be worth checking that RDM is turned off on any of the DMX buffers et al...? Depending on the make of the LED par, the cheaper units do not like RDM. Hi RDM uses the spare 2 pins of the 5-pin connector. If it's 3-pin in to the fixture, then it won't be receiving RDM. Additionally, unless the buffer is RDM capable, which I seriously doubt, then any issue will stop there.Also as it works when he is connecting it straight to the desk, again that rules out the RDM issue.Good shout though. If I was in this situation, then the only way to accurately to sniff out the problem would be to use a logic probe, DMX analyser or silly scope at the end of the run and actually see what is coming out of the end. As the unit is flickering like a good'un, I would immediately suspect that its frame corruption and you're not getting 100% of the signal though all of the time. Equipment that doesn't have a decent decoder will often accept a corrupted frame as valid and just assume its received 512 zeros rather than hold and wait for a frame that starts and finishes properly. This is especially true for chinese/generic LED stuff that have single-chip decoders with no framebuffer. Faced with that situation, it is usually quicker just to rip out of the cabling and start again rather than trying to rectify an a fault that may be difficult to find. As the others have said, if they've used crap wiring which has degraded over time, or used Cat5 on the wrong pairs, or used 1.5mm TNE or even 4-core speaker cable (all of which I've seen in 'professionally-built' venues) then you're on a hiding to nothing. All the bestTimmeh Link to comment Share on other sites More sharing options...
Oovis Posted August 31, 2018 Share Posted August 31, 2018 RDM uses the spare 2 pins of the 5-pin connector. If it's 3-pin in to the fixture, then it won't be receiving RDM. Additionally, unless the buffer is RDM capable, which I seriously doubt, then any issue will stop there. No, RDM is a bi-directional protocol over the same pair of pins. The controller sends out a packet requesting information and waits for a reply packet from the selected device, or devices. This is why RDM won’t work over dumb DMX splitters and requires ones which can send data through in both directions. Link to comment Share on other sites More sharing options...
eamon Posted August 31, 2018 Share Posted August 31, 2018 To confirm, Oovisnis correct re the use of Pin 2 & 3. The original desire was to use pins 4 & 5 but just not possible with the majority of DMX cables.More and more splitters have an RDM option than can trip you up etc. The reason I suggested it was due to the problem solving methods employed which seems to rule out the console hence looking at all items downstream. https://en.m.wikipedia.org/wiki/RDM_(lighting) Without knowing the background and equipment involved, we are speculating as to his problem and the best way to fix it. Hopefully in a helpful way! Eamon Link to comment Share on other sites More sharing options...
alistermorton Posted August 31, 2018 Share Posted August 31, 2018 I'd say the fact that it works on the desk and directly on the distro but not on the DMX outlet on the bar strongly suggests a cabling fault between the distro and the bar outlet. Link to comment Share on other sites More sharing options...
Jivemaster Posted August 31, 2018 Share Posted August 31, 2018 If the DMX is good at the splitter output then look carefully at the cables to the bar ends. Count the terminators, -one per line! Look at the cable and core structure. Does the cable say anything along it's length. Is the cable (each of them) part of a multi, is it correctly screened. Is there any connection between any core and ground/chassis/etc. If there are any odd looking three or five pin connectors, replace them with prime brand items. MAybe buy a new good long DMX lead and use it to test everything by substitution, -if the new lead works the old lead was KAPUT so BIN IT. Look at the DMX line down the bar, is it a fan out or a daisy chain? Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.