Automated Fixtures Number of DMX fixtures

[Zel]

What's the maximum number of intelligent lighting fixtures you can put on a single run of DMX with proper cable and termination? I've heard it's in the 30's but I'm wondering if anybody has hit a "hard wall". I'm using fix LED’s (35) and want to add LED movers.

Thx

 

[tdrga]

It has to do with the total "unit load" that the fixtures put on the line- the DMX spec is no more than 32 "unit loads" per run. Most fixtures are one "unit load", but there are a few that are less.
There is no "hard wall" - you might get away with 35 or 40 fixtures for a while and then suddenly they will stop working for no apparent reason.
If you have that many fixtures to control, it would be best to split them up and put them on different outputs of an opto-splitter. Each opto-splitter output counts as a separate run, so each output can have up to 32 unit loads.

Hope that helps-
-Todd

 

[chausman]

I don't think there is a "hard wall" where it will fail. It depends on the devices connected. 32 is a common "limit", but is not entirely correct. Some people prefer to limit it to about 24 to be safe.

32. (You may have chosen 24 as an arbitrary "comfortable" number.) Technically, 32 is not correct either*, but it's the number most commonly bandied about. An opto-splitter extends this limit, as well as likely makes the cable runs more convenient.

...

*

...A splitter will cut down on your cable runs. You should also take a look at the number of DMX devices you put on a single DMX run. The ANSI version of DMX says 32 unit loads. While that may not necessarily be 32 physical devices it's good practice to not put more than that on a run. You also need to make sure you have a DMX termination for the end device of each run of DMX cable. As along as all your DMX cables are up to spec. that should eliminate any random flickering of your lights. ...

 

[Zel]

OK, so would it be possible to daisy chain the opto splitter in the line and continue the run that way or do I need to split it at the head end?

 

[chausman]

OK, so would it be possible to daisy chain the opto splitter in the line and continue the run that way or do I need to split it at the head end?

You can put the opto splitter wherever in the line it is most useful. Of course, you can't put the splitter at the end of a run of 40 fixtures and hope it will work right. Most people will put the splitter where it will allow one "branch" to go to each electric/catwalk that needs DMX. It also means that if one device starts causing issues (certain Chauvets and DMX don't seem to mix well), it won't take down the entire rig, only the one "branch" of the opto splitter, which can make things easier to troubleshoot as well.

 

[Zel]

Thx everyone for your input

 

[AEGProduction]

I have hit a "hard wall" at 42 fixtures once. All LED's. It was pretty easy to diagnose the problem, then again everything was on the ground, and I didnt have to climb to add an inline driver (splitter). On that note, does anyone know of a single output line driver? Sort of like a signal booster as opposed to a (1-2/4 splt)? Haven't looked too hard, just wondering if anyone knows off the top of their head.

 

[Zel]

I have run across those before, can't remember where but the price difference wasn't worth it.

 

[chausman]

I have hit a "hard wall" at 42 fixtures once.

I should probably rephrase the statement about being no hard wall. There is a point where it just won't work, but that point depends on so many variables that we can't just say "it will always stop working at x number of fixtures". But there is a limit.

 

[derekleffew]

...On that note, does anyone know of a single output line driver? Sort of like a signal booster as opposed to a (1-2/4 splt)? Haven't looked too hard, just wondering if anyone knows off the top of their head.

From Doug Fleenor Design - DMX512 Inline Isolator :

The isolator also eliminates ground loops and provides high drive current for long control runs.

 

[AEGProduction]

I should probably rephrase the statement about being no hard wall. There is a point where it just won't work, but that point depends on so many variables that we can't just say "it will always stop working at x number of fixtures". But there is a limit.

I was not trying to say that 42 fixtures is the "hard wall limit," I was not trying to correct either. this was just my personal experience with hitting it. I'm sure others have hit it at other limits.


Thanks to derekleffew for finding that isolator for me. Looks like I can buy a 1x4 splitter for cheaper than a 1x1. Your the man!

 

[Crisp image]

Thanks again for your input. To quote a classic Australian Movie (Low Budget but totally worth it look it up and have a laugh) "The Castle" "you are an ideas man".
I will keep looking and thinking about this. Designing this sort of stuff is fun but sometimes frustrating.
Regards
Crispy

 

[Chris Pflieger]

It used to be that one RS-485 receiver (one inside every device) was equal to one unit load, but for the last decade or more 1/4, and now 1/8, unit load transceivers are the norm. Which means the theoretical limit is more like 255 devices.

 

[robmerow]

I've recently had an issue using a Chauvet Data Stream 4 with 4 branches, and one bad unit on branch 3 actually caused havoc on all 4 branches, which I previously thought to be impossible.

This made diagnosing the problem very time consuming, as I was operating under the incorrect assumption that the only thing which could affect all 4 branches was the DMX source or the splitter itself.

Is this actually possible, or it the Data Stream not a true "isolator"?

 

[Chris Pflieger]

I've recently had an issue using a Chauvet Data Stream 4 with 4 branches, and one bad unit on branch 3 actually caused havoc on all 4 branches, which I previously thought to be impossible.

This made diagnosing the problem very time consuming, as I was operating under the incorrect assumption that the only thing which could affect all 4 branches was the DMX source or the splitter itself.

Is this actually possible, or it the Data Stream not a true "isolator"?

Depends.

A good opto-isolating splitter will protect against a mischievous device on one branch. In this case I see that product is from Chauvet DJ so I expect that corners were cut - reliability is expensive.

 

[JD]

Although pins 2 and 3 constitute a "balanced" line, this is not actually so. Much like a "transformerless" microphone input on a mixer, the signal hits active electronics that need to have some form of ground reference. Because of that, pin #1 connects to the "electronics" ground or common (not to be confused with the electrical frame ground.) The result is that every connected unit adds a little bit of its own electrical noise to the line. If there are units that connect frame and electrical ground (some do), then you can also pick up ground loops as well. This cumulative noise can be as big a factor to limiting the number of units in a loop as the RS-485 unit loading. It is also why opto-isolators are so great. The break the ground noise loop. BUT BEWARE! Many of the less expensive ones only isolate the input from the output, but DO NOT isolate the grounds between the outputs. That's because you would need a separate power transformer for each output leg. In doubt? Measure pin one on one output to pin one on another output. True isolation = open circuit.

 

[Jay Ashworth]

I should probably rephrase the statement about being no hard wall. There is a point where it just won't work, but that point depends on so many variables that we can't just say "it will always stop working at x number of fixtures". But there is a limit.

Nope; you're right.

"Hard", here, implies that the number is hard: that over that number always won't work, and below, it always will.

Serial addressing might cause such a limit, for example. Current loading, generally, won't, the limit is "soft".

 

[RickR]

It used to be that one RS-485 receiver (one inside every device) was equal to one unit load, but for the last decade or more 1/4, and now 1/8, unit load transceivers are the norm. Which means the theoretical limit is more like 255 devices.

Do you have any hard data on fractional fixtures? I've had a hard time confirmation from manufacturers.

 

[Chris Pflieger]

Do you have any hard data on fractional fixtures? I've had a hard time confirmation from manufacturers.

I'm basing this on the fact that RS-485 transceivers from integrated circuit manufacturers (Fairchild, Texas Instrument, Linear, etc.) are, for the most part, 1/8 unit load. The 1 unit load models are legacy units running at 5 volts; my guess for qualified applications where changing the spec is really expensive due to re-certification.

I haven't used a full load transceiver in the last 15 years - and those were all 5V DIP packages. There's no reason a new design wouldn't be a 1/8 unit or maybe a 1/4 unit load.

 

[EdSavoie]

While the chips themselves have a small load rating, i wonder if the PCB traces mangle the signal at all.