DMX Termination For Dummies

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Hi Guys. I am really not getting this whole DMX termination deal. I have looked through a bunch of threads and it is not getting through my brain...

Can you please give me a plain and simple (explain it to me like I'm a caveman), what does DMX termination do, and how do I do it?

When people say "put a 120 ohm, 1/4 watt resistor across pins 2 & 3," what does that mean?

Thanks!

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cutlunch

Active Member
Hi. The 120 ohm resistor needs to be soldered into place. One end of the resistor is soldered onto pin 2. The other end is soldered onto pin 3. Being a resistor it doesn't matter which end goes to which pin. The numbering of the pins is normaly marked on the plug. Lokk at the output plug of what you want to terminate. Probably labeled "DMX Ou"t to see which type of plug you need a female or male. JUst make sure that the metal leads of the resistor can't short against each other, pin 1 , or the metal case of the plug. Use heatshrink tubing or insulation tape to ensure this.

With termination the main thing to rember is that the terminator is the last thing in the DMX line. Eg if you have four dimmer packs and the DMX goes in the first then out to the second etc until the fourth. There won't be a lead coming out of the DMX Out on the fourth pack. This is where the terminator goes.

Just remeber some equipment comes with a builtin terminator. These normaly have a switch to put in / take out the terminator. Check your equipment manual to tell whether there is an inbuilt terminator.

Just remember that at the end of any DMX line there should be a terminator.

Hope this helps.

JD

Well-Known Member
Ever hear someone shout in a gym? Hard to hear them because there is so much reverberation. If the same gym was carpeted and curtains were hung, the reverberation would be suppressed and you would be able to hear better. The rugs and curtains would absorb the sound.

In a way, that's what the dmx terminator resistor does. Without it, the signal "echoes" about the wiring making it hard for the dmx devices to hear it well. (ok, bit of a stretch) The terminator kind of absorbs the signal at the end of the loop suppressing all that electronic echoing. Note I say "kind of" and "bit of a stretch." The actual nature of what is going on is a bit more complex, but I think the "gym" analogy works pretty well. Hope this helps.

TuckerD and DarSax

Active Member
Thanks Guys.

I got a 100 ohm 1 watt resistor with 5% tolerance. Is this fine?

Jezza

Active Member
In theory, it will help, but I wouldn't recommend it. 120 ohms is what is prescribed as being the correct resistance to use to terminate a DMX line. The resistors are maybe a few cents at any computer/electrical supply place. It shouldn't be an issue tracking one down.

Van

CBMod
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Thanks Guys.

I got a 100 ohm 1 watt resistor with 5% tolerance. Is this fine?

Go to Radio Shack, they still carry resistors, < in most stores anyway. a 100ohm resistor is not going to hurt or adversly effect the performance of you system, but is best to get the right equipment.
To further the analogy if you are getting carpeting and drapes for that gym your'e shouting in and you need 120 square feet <120ohm resistor>but you've only got 100 square feet of soft goods , <100 ohm resistor> you can use what you got but it's not as effective. < god this is a crude analogy, but helpful> Now if you double the size of the gym < add more nodes/runs onto the DMX line> and you use 100 square feet of carpet again your now going to have twice as much reflection of noise. That make sense?

Active Member
Go to Radio Shack, they still carry resistors, < in most stores anyway.
I went to radioshack and they only had the 90s, 100s, or 150s. I have already finished making it. Would a 150 ohm resistor work?

Van

CBMod
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Hmm it's like splitting hairs. Too much resistance and then the good signals get absorbed. If you could find them you could put two 60's in series, or a 100 and a 20 etc. it just math. Do not, however put them in parallel this will only increase the wattage handling capability. Try parts express online, or some other electronics suppliers. Frys electronics, or ... do you have a local radio supply store, or computer network supplier? The 120 is used a lot in those apps as well.

stantonsound

Active Member
I am not one for buying things that are easy to make. For instance, I would never consider buying pre-made stage pin cables, or twofers (socapex is a different story, that is a pain in the butt to make), but dmx terminators are only about $4-$5. I stock these with my rental gear, as they are more likely to get returned in they look like something important, rather than looking like a broken dmx or xlr connector. If it is going to take you hours of searching and driving (with gas at \$3+ a gallon), just go online and order one.

JD

Well-Known Member
OK, some quick notes on resistors- The higher the number, the less the load. so, two 220 resistors in parallel will give you 110 ohms. Two 60 ohm resistors in series will give you 120 ohms.
(Ohm's Law R=E/I Resistance. Resistance = voltage divided by amperage flow. A light bulb drawing 1.2 amps at 120 volts would be introducing a 100 ohm load. Watts is volts times amps, so our fictitious light bulb would be dissipating 144 watts.)
Radio Shack stinks for parts these days. If you need a bunch of other stuff, try MCM Electronics. http://www.mcminone.com/search.asp?fkey=&restrict=3829507&keyword=120+ohm If the link works. part number 34-120 will give you a bag of 20. Problem is, unless you have a bunch of other stuff to order, the S&H will kill you.

gabe

Member
Also when in parallel, (1/R1) + (1/R2) = (1/RT)

Active Member
To further the analogy if you are getting carpeting and drapes for that gym your'e shouting in and you need 120 square feet <120ohm resistor>but you've only got 100 square feet of soft goods , <100 ohm resistor> you can use what you got but it's not as effective. < god this is a crude analogy, but helpful> Now if you double the size of the gym < add more nodes/runs onto the DMX line> and you use 100 square feet of carpet again your now going to have twice as much reflection of noise. That make sense?
What happens if I use a 150 Ohm resistor instead of a 120 Ohm resistor?

Van

CBMod
CB Mods
What happens if I use a 150 Ohm resistor instead of a 120 Ohm resistor?
you'll have 30 ohms that are ohmless........ rimshot please.

Probably nothing, at first. it will eventually get into the thesoteric areas of electrical theory, that I'm sure several people here are more versed in than I. However. It would probably work... for one terminator, but more than one or if it's installed on the end of a long dmx run etc. the resistance could become great enough that that it will essentially "suck up" the signal. Imagine it's kinda like a car engine a little resistance on the exhaust is a good thing and keeps things running smoothly, too much resistance and the power of the engine is used up trying to push out the exhaust. Too little resistance and the engine will run choppy. That'a a really horrible analogy... Ok let's say your'e trying to blow out a candle. You have to blow through a straw to do it though. first you get one of those really tiny stir sticks, like you get at the coffe places. You have to blow really hard to get air through but there's not enough mass of wind going to blow out that flame. Then you get a piece of 3 inch pvc and blow through it. No Resistance, but the air isn't moving fast enough to blow out the candle. Then you get a good old fashioned soda straw, just the right amount of resistance, candle goes right out. Now does it make a big difference whether you use a soda straw or one of those Mondo Big gulp straws? No, not really, but electronics are a bit more sensitive than a candle flame. Man the analogies are really sucking tonight.. or is that blowin'

Really I'm not trying to be pedantic I just don't know how better to explain it. It's one of those, " Yeah, it'll work... in a pinch." it's just best done right.

icewolf08

CBMod
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Van, are you trying to say that there is a right way to do things and a half-arsed way?

You don't just do something the wrong way because that is what you have the parts for, you get the right parts.

Van

CBMod
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Van, are you trying to say that there is a right way to do things and a half-arsed way?

You don't just do something the wrong way because that is what you have the parts for, you get the right parts.
Yeah, what he said.

But you gotta admit the straw thing was kind of a cool analogy sorta

icewolf08

CBMod
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Yeah, what he said.
But you gotta admit the straw thing was kind of a cool analogy sorta
If it makes you feel any better, I enjoyed reading those analogies

cutlunch

Active Member
I just gotta get into this analogy story telling.

When I was studying electronics we had to study all this, frequency, wavelength, impedance matching.

I remember a tutor telling us this story.

In a country with a less then honest government an outside company was asked to build an electric railway/tramway.

For some reason this system involved an AC supply instead of DC.

The system was built and ready for commissioning when the government said " We're not going to pay you what we owe. And we are going to kick your engineers out of the country"

Deciding on revenge one of the engineers cut the wires on the system so that they were exactly cut in multiples of the wavelength. At this point if the load is mismatched more energy is returned down the line to the source. Ie for simplicity say wavelength = 3 units long then he cut the wires so they = 9 units long. Wavelength in metres = 1 / frequency in Hertz.

Then comes the big the day, brass bands, politicians etc.

They haven't got the train connected to the wires yet. The top guy turns on the power. The power goes rolling down the wires and when it gets to the end there is no load to take it. ( no matching impedance). So the power says I am out of here and heads back to the power generator. Because there is now more power at the generator then it was designed for it blows a fuse. So some bright spark puts in a heavier fuse and they try again. Same thing happens then a bigger fuse. They try again but now there is no protection and the generator destroys itself. Last laugh to the engineer.

In your case with a 150ohm instead of 120 ohm nothing will blow up. It just means that some of the signal can bounce back down the line mixing in with the dmx signal coming out of the DMX source. It may do nothing or it may just change the value of a chanels data by a couple of bits thus causing a flicker or something unexpected.

As has been said it might have little effect on short dmx runs but on longer runs with more dmx devices connected problems might start appearing.

But I definitely would loose sleep over it.

Ham radio operators do worry over this. They have meters that show how much energy sent from the transmitter is not going out the antenna due to mismatched impedance. If too much power is being wasted because of a mismatched impedance (anntena load) you may need a bigger transmitter. The bigger transmitters is needed to send the radio signal the same distance as a smaller transmitter that has a better matched impedance ( antenna load).

Some one just to go and add some science to this. He.. He..

Active Member
In your case with a 150ohm instead of 120 ohm nothing will blow up. It just means that some of the signal can bounce back down the line mixing in with the dmx signal coming out of the DMX source. It may do nothing or it may just change the value of a chanels data by a couple of bits thus causing a flicker or something unexpected.

I thought that's what happens without a terminator .

Whatever... I am putting a 100 ohm resistor and two 10 ohm resistors in parallel and it measures out to 18.9 ohms on my ohmmeter. Around as much resistance as I would get with a 120 ohm resistor with 5% tolerance. Finally... Just need to get all of the projectors and stuff off of my work bench so I can solder!

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