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light goes on when board turns off

Discussion in 'Lighting and Electrics' started by FlashBang, Apr 7, 2011.

  1. FlashBang

    FlashBang Member

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    Hi,

    I am currently using an ETC Express 24/48 with ETC L86 dimmer rack.

    When I turn off the power switch on the back side of the console, often there is a stage light (seemingly at random) which jumps to full power, and then fades at the same rate as the console LEDs.

    Any ideas?

    Thanks in advance.
  2. icewolf08

    icewolf08 CBMod CB Mods

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    Sounds like an errant data spike on the DMX line when you turn off the console. Try unplugging the DMX before you turn off the console and see if you have the same issue. If you are only using one universe of DMX you could try configuring the second output on the Express to be 1-512 and then plug your DMX line into the second port and see if you have the same issue as well.
  3. ScottT

    ScottT Active Member

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    Are you using a DMX to other protocol converter?
  4. ptero

    ptero Member

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    One theatre I design at has the same issue with this console. The ME's solution is to turn the console back on, then off again and it usually goes away. I understand it happens now and then for them - couple times a month? Again, not sure why.
  5. JD

    JD Well-Known Member

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    Cool! (as far as problems go) This is a real mind tease. The slow fade indicates a series of new and descending DMX values are being written to the channel.

    If the channel was programed to "hold last value" when DMX was lost, one could see a light coming on and staying on due to noise on the DMX line at shut down. This is not the case.

    If the channel was programed to "go to black" when DMX was lost, then it may flicker on noise and immediately go to black. Not the case again!

    This is reminiscent of the old analog days where the DC voltage would slowly fade as the caps drained, much like the LEDs on the board, but in this case, the signal is digital, and something is encoding the value as it drops.

    Sounds like a board or encoder issue. Let us know!
  6. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    See this thread: Lights turn on at shutdown - Electronic Theatre Controls from the ETC forums. Also this post. BTW, I love the term "DMX death gasp", coined by Jason Badger, a top programmer.

    Not necessarily. I think the dimmer fading at the same rate as the console LEDs is purely coincidental. The L86's "DMX Loss Behavior" is to fade all in 3 or 5 seconds, I think.
    Last edited: Apr 7, 2011
  7. JD

    JD Well-Known Member

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    A programed fade from last valid value sure does explain it!

    One would have to equate this to the turn-off thump in audio equipment.

    This is mitigated in modern equipment by a circuit that clamps the output on loss of supply. You would think the same would be easy to incorporate into a DMX driver circuit. Instead, it sounds like the output of the driver is pulsing high on turn-off, and being interpreted as a string of 1's. This would hit whatever address was in the DMX string at the time, and be interpreted as values of 255.

    The design rectification would be to clamp to the low, or 0 value at turn off. Unfortunately, easier said then done. To achieve a "0", the + data must be low, while the - data must be held high. This state must be held through several frames in order for the dimmers to acknowledge loss of DMX. (Less than one second.)

    As DMX is a differential signal, simply bringing both outputs low would be read at random, the - line must be held high for a bit.
  8. avare

    avare Member

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    Is the DMX line properly terminated?

    Andre
  9. sk8rsdad

    sk8rsdad Well-Known Member Premium Member

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    I believe the DMX transceiver (transmitter?) IC used in the Express starts streaming from the instant it gets powered up until the instant it loses power. If so, it is very likely that stray noise can cause an inadvertent light or two to come on. We used to see this behaviour back in the old days before switching over to SACn to talk to our Sensor racks. There are likely a few threads here at CB on the topic.
  10. DavidNorth

    DavidNorth ETC Rigging General Manager Premium Member

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    Yes, Derek and JD are correct. This last gasp DMX blurb can be sent from most consoles on the market and any dimmer rack or pack with a Hold Last Look feature can capture it and display it as the OP has noted. JD is also right that stopping this from occurring in product design is very difficult.

    What happens is that as the power supply in the console fades away after power shutoff, the DMX driver inadvertantly sends out some junk in the form of possibly recognizaable data right before turning itself off. The dimmer rack accepts that junk as valid data and then holds the level until the time it's suupposed to time out and fade out.

    Every console does this differently and even two Express consoles might perform differently. We even had some people call us when they moved from Express to Ion that suddenly starting having this same issue when nothing else in the system changed.

    We are coming out with a way to disable to chip and hopefully stop this kind of power down blip. The Ion series of consoles will benefit from this first. Keep an eye out for more info.

    David
  11. FlashBang

    FlashBang Member

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    Thanks for all the in-depth feedback!
  12. JD

    JD Well-Known Member

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    Ah! A new product for DFD to market! A DMX "mute" box.

    It could go inline and run off a wall-wart. Hit the switch and it just starts outputting all zeros! Would be a lot cheaper for most people than changing or upgrading the board. Or better yet, it could cue off a certain number on a certain address. Program that as the last scene before you power down. (reverse on power up.)

    Wonder if his passive A/B switch would work?
    http://www.dfd.com/sw.html
    Last edited: Apr 8, 2011
  13. DuckJordan

    DuckJordan Well-Known Member

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    Ah... Your over thinking it.. I think wiring a 3 pole switch with one pole having nothing connected for (off) and basically a pass through on the (on). I forsee it being a lock switch with it in a wall plate so it can also be used as a system lock out.
  14. TimMiller

    TimMiller Well-Known Member

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    Y'all are all over thinking it what I always do is unplug the dmx cable before turning off the console. It's a real pain when you power down a moving light rig then part of the rig decides it's going to fire up when you turn off the console. I have seen this problem in the express and hog II consoles. This problem is going to start going away as eithernet starts to take over such as in a hog III where the console does not output any dmx.
  15. GoboMan

    GoboMan Member

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    Lights Coming On After Board Is Powered Down

    I have run the light board for many shows at my theatre, and every now and then when I shut the board down (ETC Element), one or two random lights will turn on immediately after the board shuts off. Since the board is off, there is no DMX signal being sent, so I am wondering how the lights are even getting their power. After about 5 minutes, the lights will slowly fade themselves out and all will be good after that.

    What is causing this to happen? We have 38 dimmers powered by two ETC Sensor dimmer racks (one is a large rack that holds 26 dimmers and the other is a smaller, 12-channel dimmer box). Is it anything I should be concerned about?
    Last edited: Jun 25, 2013
  16. Les

    Les Well-Known Member Premium Member

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    Re: Lights Coming On After Board Is Powered Down

    It's called the DMX Death Gasp. It's really nothing to be worried about, and there really isn't a way to stop it short of unplugging the DMX cable from the board before powering down. The thought process as to how this happens is that consoles sometimes send out an errant bad packet of data at power-down, which is sometimes readable by the dimmers as a valid command. The lights continue to receive power for as long as the dimmer rack wants to supply it -- a continuous DMX signal does not necessarily need to be present for this to happen as all DMX does is say "hey, dimmers, do this". This may happen more than you realize but it goes unnoticed if the channel inadvertently brought to full is unoccupied. You may be able to tweak the amount of time that the dimmers hold the lights on by changing the "Hold Last Look" setting, but that is generally ill-advised as most would rather deal with the occasional ghosting channel than have the potential for a console failure (or unintentional reboot) to plunge the stage in to darkness mid-show. Kind of an extreme solution to a minor problem.

    From this thread: http://www.controlbooth.com/forums/lighting-electrics/23981-light-goes-when-board-turns-off.html

    See also: http://www.etcconnect.com/Community/forums/t/4018.aspx?PageIndex=1
    Last edited: Jun 25, 2013
  17. GoboMan

    GoboMan Member

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    Re: Lights Coming On After Board Is Powered Down

    Thanks, Les, for your helpful answer and directing me to more posts about this issue! In reading through some of the posts on the ETC Connect forum, someone mentioned that it is a good idea to turn off the dimmer racks between performances. At my theatre, we have always left our dimmer racks powered up 24/7, and as long as I have worked there we have never turned them off between shows. I was never even told by the TD to do that. Whether it would help prevent the Death Gasp or not, is it bad to leave the dimmer racks on all the time?
  18. Les

    Les Well-Known Member Premium Member

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    Re: Lights Coming On After Board Is Powered Down

    You're welcome for the answer!

    No, it's not bad to leave the dimmer racks on at all. Most installed dimmer racks are powered up 24/7 -- some are even actively used 24/7. When your theatre is 'dark' (between shows), the dimmer racks will go in to a "hibernate" mode where nothing really runs. The CEM will be slightly active so as to listen for any incoming DMX, but this is not a problem. Besides, if your houselights are powered through the dimmer racks, you will want to have them available for emergencies, cleaning crews, miscellaneous use, etc without having to throw a breaker. In fact, most large breakers such as those that supply dimmer racks really aren't designed for regular, everyday use.
  19. GoboMan

    GoboMan Member

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    Re: Lights Coming On After Board Is Powered Down

    Awesome, thanks! We are a small black box style theatre and the only thing our dimmers control are the stage lights...our houselights are controlled by simple wall dimmer switches in the control booth, but I will continue to leave the dimmer racks powered up. Thanks, again! :)
  20. Les

    Les Well-Known Member Premium Member

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    Re: Lights Coming On After Board Is Powered Down

    No prob!

    I will mention that when I worked in a space with a blackbox, we would switch off the dimmers (which were 12ch sensor "portable" racks) between shows. Of course, we only had about two shows in there a year, so that in-between time could be 6-12 months. I agree that they should probably just be left on unless you know it's going to be a few weeks or months before they're used again.

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