Control/Dimming DMX Problem

Ghostman21

Member
Joined
Oct 11, 2008
Hello everyone! Long time lurker first time posting. I just took over a new theatre as TD and ran into a very odd problem with their DMX control system and was wondering if someone could help. The problem is that when you plug the console (Expression 3) into any of the DMX input ports in the theatre. All the lights go crazy flickering and flashing at random. BUT when you run into any of the ports by first running the DMX through a DMX splitter we don't have that problem. I remember running into this problem once before years ago and I seem to remember that the solution is pretty simple but I can't for the life of me remember what the solution was. Any suggestions? Thanks!
 

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Nov 24, 2005
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Its being caused by the output of the console, when you put in through the splitter, it cleans up the out put in a sense. Try this, go to setup and then output settings. Slow down the DMX output refresh rate. That should fix it.
 

mbandgeek

Active Member
Joined
Apr 1, 2006
Location
North Carolina
I do not know all that much about DMX, but could it be a grounding issue? I have heard of that causing some crazy things to happen with DMX signals.
 

Ghostman21

Member
Joined
Oct 11, 2008
Hmmm.....It might be a grounding issue. I'll have to investigate. The problem is that the theatre was built in the late 60's and has been renovated and refit twice in it's lifetime. With each renovation things were taken off or grafted on to all of the technical systems and sometimes in between as well. It makes it difficult to know where anything is run or how many times it was spliced between point A and point B. I've already found two splices in just one of the runs between the lighting booth and the dimmer racks. I'm trying to figure out where the DMX comes into the Teatronics MD288 rack. I'm gonna try to run the console directly into the racks and see if maybe the problem is simply that the control cards are so old they're not fast enough to translate the DMX properly even at slow transmission speed.
 
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waynehoskins

Active Member
Joined
Jul 4, 2008
Location
Arlington, TX
The local high school (and until a few years ago, the university) had MD288 racks driven by Express-series consoles with no problem, so it's not an intrinsic design flaw of the firing cards in the rack not being to make sense of Expression-era DMX512 (though there is a generation gap there).

You might consider, though, lowering your address count (dimmer count). I don't know if that would make a difference with the refresh rate, but it might stand a chance.

And then it could be just a sloppy RS-485 driver in the board that the opto cleans up for you. Could also be that the RS-485 line you're talking into has too much load (double terminated or too many devices) for the driver chip in the board, but the opto's outputs have a higher-current driver behind them.

In any event, if you can run the board directly into the rack, you should have no problems; if they persist, I'd really suspect the RS-485 transmitter circuit in the board (probably the oscillator if I had to make a WAG).
 

Ghostman21

Member
Joined
Oct 11, 2008
And then it could be just a sloppy RS-485 driver in the board that the opto cleans up for you. Could also be that the RS-485 line you're talking into has too much load (double terminated or too many devices) for the driver chip in the board, but the opto's outputs have a higher-current driver behind them.
I'm inclined to think that this is the problem. As near as I can figure the system has the DMX running into the MD288 rack without connecting to the control circuits in the rack. Instead it seems as though they spliced the DMX and ran it back out of the rack and into a Unison controller which then sends the DMX back into the MD288 rack and actually connects to the control lugs. The DMX then comes out of the MD288 rack and goes into two Teatronics ComStar (12 dimmer) portable racks which has the control daisy chained from one rack to the other. (I don't know whose crackerjack idea this was but when I find them I'm going to hit them!) At this point I'm wondering if my best bet might just be to pull new DMX runs through the walls to clean up the splices and get good clean runs from one end of the system to the other.

On a totally unrelated note. The Unison control module is one of those 24 dimmer type racks but as far as I can tell the ComStar racks and the unison control were installed at the same time. Wouldn't it have made a whole lot more sense just to use the rack space in the unison controller for dimmers instead? If the unison was installed later wouldn't it have made more sense to just replace the comstar units? I know it would have added to the cost but still! *Shrug*
 

PhantomD

Joined
Mar 3, 2006
Location
Brisbane, Australia
I'd say you have a wiring problem somewhere definitely.

Consoles that output at the full DMX refresh rate can really expose problems in older building wiring that don't cause trouble with (normally older) control boards that don't refresh at the full rate.

That's why people buy fancy new control desks and complain when the rest of their system falls apart!!
 

Malabaristo

Well-Known Member
Joined
Jul 11, 2008
Location
Wisconsin
There seems to be some confusion about how an opto-isolator affects a DMX signal. The vast majority of them (all that I know of) are simple repeaters: whatever data you put into it is exactly what you get out of it. There's no buffering and regenerating involved. If there was an issue with the refresh rate of the console, adding an opto would not make any difference since you'll get data out of the opto at exactly the same rate.

Given the state of wiring in this particular facility, that would certainly be my first place to look for problems. It very well could be a grounding issue. If that's the case, then leaving the optoisolator connected is probably the simplest solution for you. You may also have the Data+ and Data- wires swapped somewhere, or one of them may be disconnected. Those problems would be less likely to be fixed with an opto, but they're something to check for.

You may also have a marginal DMX transceiver chip in the console. Have you tried using either of the other outputs? The transceiver chip is a 75176 (available from Digikey and others) and it's quite easy to replace on the Expression 3. Feel free to PM me if you want further instructions or pictures.

As with any problem involving ETC gear, your best resource is their phone support staff. They'll be happy to talk you through any problem from 8-5 central time on weekdays, and have someone available 24hrs for emergencies:

1-800-688-4116
 

dramatech

Well-Known Member
Joined
Mar 12, 2008
Location
Winter Haven, FL
It is very possible that you have a grounding problem caused by pin one tied to the shell of the connector. Early on in DMX/RS485, it was considered correct to tie pin one to the shell like was done in early audio wiring. In DMX, never have the pin one tied to the shell. An optoisolator lifts the electrical ground and the data lines are also lifted electronically. The DMX signal is strictly regenerated from light pulses.
When the splitter solves the problem, it indicates that there is electrical interferance, most likely the ground. Check all of your connectors for proper wiring.
 

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