Hopefully not a silly question... ? DMX line termination.

[Snowcat]

Evenin' all. Have been following the forum for a while now and I'm curious about something that'll probably seem stupid.

Why the need to manually terminate the DMX line?

I've spent the last 7 years or so mucking around with mid sized rigs (100 channels or thereabouts) and I've never needed to terminate the line or have had any problems related to a non-terminated line. Have I just been lucky all this time (now jinxed of course ) or can fixtures themselves act as DMX terminators?

Thanks for the patience as always.

[remco_k]

Google would have the answer of your question. There are many pages out there describing why you should terminate the DMX line.
In short; if you don't terminate the DMX line then the transmitted data on the line could have ghost like effects. (The data bounces back at the end of the line to the beginning and then bounces to the end, and to the beginning (and so on), each bounce slightly shifted in time, and a little less).
This ghosting gets worser on longer lines, and worser with more fixtures. If using bad cabling, the gosting is even much worser.
Next to that, an unterminated DMX line is a receiver and will get interference from other electrical equipment. This could be devistating, together with ghosting.

At some point the ghosting and interference gets the overhand. First at occasional times you notice that a few fixtures don't respond or are doing the weirdest stuff. Or they just go on while you are sending nothing.
When ghosting is worser, nothing works okay anymore. And the fixtures can't 'decode' your real DMX signal anymore.
Some fixtures have a perfect DMX interface build in, capable of reading the DMX data correctly while ghosting is heavily there.
Other fixtures stop working immediately or go random runs when ghosting kicks in just a little bit.

When using pretty short DMX lines (few meters to 30 meters) proved me to be working while not terminated. But occasionally I notice a light flashing once when somewhere else an airconditioner switches on, or some other electrical device...
Yet, that is no guarantee that it stays that way, not even in fixed installations. Thats why you should always terminate the DMX line. A terminator costs like nothing and can be made by hand easy for almost nothing. So don't take ANY risk EVER! It is actually possible (theoreticly) that some fixtures seem to work okay but when the event is busy they can just decide that they go random or stop forever.
Because of your unterminated DMX line, the line receives more interference from other electrical devices (cellphones, airfans, airconditioning, and lots of other electrical devices...) these can actually make your DMX line unusable during the event, because those where not there (or on) when you installed and tested your equipment.
And then, there you are with your light controller, unable to put in the terminator (if you have one) because its like 10 meters above 250 party people. Maybe none of your fixtures react anymore. Small chance of this, but it could. Then, you have a free night.

Some fixtures terminate automaticly when their DMX output XLR stays empty. I hate that by the way, because people could leave the DMX termation out thinking that it is terminated automaticly, while its not (in case of defect or human mistake).
Next to that, if that mechanism fails for some reason and terminates the line at a point where it should NOT, then there will be big problems thanks to that.
I prefer the DMX termination connector in my pocket (including one spare - I lose them all the time somehow) and use that.

Edit:
Forget to answer your question:
So yes, you are lucky that it works and stay that way. (As was I, but I do use the terminator now - I take no risk).

[Snowcat]

Aaah right. Had a feeling it was something to do with interference.

I've always tended to be in amongst the metal work either of old buildings or temporary exhibition structures. The "Faraday cage" effect is likely what's saved me in the past from random RF energy causing havoc. I tend to keep my signal and power lines far apart as well, an old habit from the audio side of things.

Having said that it's probably about time I started doing this properly... *Digs out the soldering Iron and compo box.*

[Tom]

remco_k has hit it on the head but I couldn't see him saying what a terminator does.

Basically, it make the fixture think the DMX line is of infinite length.

[remco_k]

You both have it right. I always tell it this way:
The terminator just 'swallows' the DMX signal completely, it goes in, but never gets out. As simple as that. Although the real technical backround about cable impedances is different. People do get this...
Some people refer that as infinite cable length, others say its a signal stopper.
Point is, the result is exact the same: signals go in, but never come back.
When not using a terminator, but an open end, thats not the case and reflections will be there.