Monoprice DMX Cable wiring
- Thread startercvhstech
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TuckerD
Well-Known Member
Cool use of the sheild as the Common / Gnd.
As far as impedance / resistance there is a strong relationship between the two (in fact the both use the same unit of measurement, the ohm). The way it was explained to me was that the characteristic impedance of a data line is the resistance of the line if it was of infinite length (yay calculus). When you have a wave traveling in a medium and suddenly the the medium is interrupted (in our case by the end of the cable) there is a reflection. By adding the resistor between the two conductors you make the cable to have the same resistance as one of infinite length and the waves can never reflect off of the end of the run because "it doesn't end". Really, this is the subject matter of upper level EE classes at a 4 year uni. But that is the lame man's definition that was given to me. So I think the two are pretty interchangeable.
Not quite. Cat 5e characteristic impedance is 100 ohms and RS485 cable is 120. The DMX Spec (ANSI 120) says that you need to use a cable with characteristic impedance of 100 to 120 ohms.
@RickR Your comment doesn't make a lot of sense to me. It's my opinion that if it's reasonable to pull another cable (which is pretty much always in my mind) that you should pull another cable. In the case that you gave it seems like it would be reasonable to do so. The only time that I have a hard time debating that is when you run into a foreign house with your show and suddenly they don't have enough DMX with a quick load in, as was mentioned in this thread earlier.
Your right that it certainly seems easier to just build some Y's and call it good but to me if you don't have to cut corners you shouldn't. Especially since it was pointed out earlier that a pair of two Y's (which is what would be required) is 3 times the cost of one new cable because of the number of required connectors. And the cost of the cable is pretty small if not negligible. Here is a 1000' of cable that meets spec for $50. http://www.amazon.com/dp/B0092TG310/?tag=controlbooth-20
EDIT: I should add that I understand that people do what they have to, and that no matter what I say I can't change the fact that it is possible to add another universe to a typical 5 pin DMX cable.
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Umm, if you factor in the cost of labour to run an extra cable in an installed setting, then the Y adapters are probably going to range between somewhat cheaper and WAY cheaper...
TuckerD
Well-Known Member
I'd like to explore that in a little bit more detail. I could be way off here because I don't have real industry experience, nor do I really know how staff wages vary by location / house size. So forgive and correct me if I turn out wrong.
I looked up the employment statistics for set and exhibit designers and for sound engineering technicians on the Bureau if Labor Statistics website. Unfortunately I couldn't find any info on electricians for the stage specifically but I am going to guess it isn't very much difference.
http://www.bls.gov/oes/current/oes271027.htm
http://www.bls.gov/oes/current/oes274014.htm
The rest of this analysis assumes a few things:
So you are only a little bit off by these calculations and I wouldn't imagine that anyone will really make the decision over $6. But I would like to recalculate the cost of labor if you were in the first or third quartile of earners.
If you were in the first quartile of earners making $16.35 an hour then the cost of the Y method is 45.56 and the cost of the new cable method is 35.65
If you were in the third quartile making 32.82 an hour then the cost of the first is 53.81 and the second is 52.82, making it still slightly (and IMO insignificantly) cheaper.
So that's the cost of labor. At some theaters it's worth it. In Billings, MT I would choose the new cable run every time because there aren't very many people who would really know when the Y is appropriate and what it does. Also the cost of labor here is pretty low. I don't work for pay and the very small number of technicians who work full time (which I can count on one hand) either don't have the expertise to know anything about DMX or don't make enough money to make the labor savings worth it.
Here is another major issue with the DMX Y. It's hard to know exactly which cable pairs are used for what on which manufactures. On some devices they are test ports for manufacturing, applying a signal to them may have unknown consequences. On other manufacturers they aren't connected, because why would they be. So you have to be careful about where you install the Y and know that it isn't a "plug into anywhere to make two universes" device. Or (the worse situation) it isn't a DMX break out that will let you to split a run in two directions. To the community theater or small church that's exactly what it would look like, and those are the kinds of people who I see typically asking these questions.
I looked up the employment statistics for set and exhibit designers and for sound engineering technicians on the Bureau if Labor Statistics website. Unfortunately I couldn't find any info on electricians for the stage specifically but I am going to guess it isn't very much difference.
http://www.bls.gov/oes/current/oes271027.htm
http://www.bls.gov/oes/current/oes274014.htm
The rest of this analysis assumes a few things:
- The median salary / wage of the person doing the install is $50,000/yr or ~ $23.03 / hr. I choose this because it's a nice number which is a bit higher than the same figure for set / exhibit designers for performing arts companies given by the BLS. (Link 1)
- The cost of a 5 Pin Male XLR connector is $5.00 if you buy more than one at a time
- The cost of a 5 Pin Female XLR connector is $6.80
- Cat5e cable costs $0.05/ft from Amazon
- $12.02 Labor
- $27.20 Female 5 Pin XLR Connectors (4x)
- $10.00 Male 5 Pin XLR Connectors (2x)
- $00.20 4' of Cat5e (12" tails)
So you are only a little bit off by these calculations and I wouldn't imagine that anyone will really make the decision over $6. But I would like to recalculate the cost of labor if you were in the first or third quartile of earners.
If you were in the first quartile of earners making $16.35 an hour then the cost of the Y method is 45.56 and the cost of the new cable method is 35.65
If you were in the third quartile making 32.82 an hour then the cost of the first is 53.81 and the second is 52.82, making it still slightly (and IMO insignificantly) cheaper.
So that's the cost of labor. At some theaters it's worth it. In Billings, MT I would choose the new cable run every time because there aren't very many people who would really know when the Y is appropriate and what it does. Also the cost of labor here is pretty low. I don't work for pay and the very small number of technicians who work full time (which I can count on one hand) either don't have the expertise to know anything about DMX or don't make enough money to make the labor savings worth it.
Here is another major issue with the DMX Y. It's hard to know exactly which cable pairs are used for what on which manufactures. On some devices they are test ports for manufacturing, applying a signal to them may have unknown consequences. On other manufacturers they aren't connected, because why would they be. So you have to be careful about where you install the Y and know that it isn't a "plug into anywhere to make two universes" device. Or (the worse situation) it isn't a DMX break out that will let you to split a run in two directions. To the community theater or small church that's exactly what it would look like, and those are the kinds of people who I see typically asking these questions.
Australia has one of the highest labour costs in the world 
The estimate for an hour to run a new cable seems low. You've got 10 mins to terminate it. But the moment you need to go into a ceiling, under a floor, or fish it through conduit through several rooms etc. it adds up quickly. Plus the fact you would normally use 2 people to pull cable...
And one would normally not use UTP Cat5e for an installed run unless inside metallic conduit - which is far from the norm here, so you're looking at STP at a higher cost.
Cat5e sounds like the wrong answer for making tails also - it's too thick to get 2 into a boot and too inflexible for my liking, but that's personal preference...
The estimate for an hour to run a new cable seems low. You've got 10 mins to terminate it. But the moment you need to go into a ceiling, under a floor, or fish it through conduit through several rooms etc. it adds up quickly. Plus the fact you would normally use 2 people to pull cable...
And one would normally not use UTP Cat5e for an installed run unless inside metallic conduit - which is far from the norm here, so you're looking at STP at a higher cost.
Cat5e sounds like the wrong answer for making tails also - it's too thick to get 2 into a boot and too inflexible for my liking, but that's personal preference...
techieman33
Well-Known Member
That's way to simple of a breakdown. The time it takes to run a new data line isn't a sure thing. It could take one guy an hour, or it could take 5 guys 2 days to run new conduit, drilling new holes in walls and floors, removing, and reinstalling equipment that gets in the way, etc. It can get messy fast. That's why when you see threads of people asking about what to do in their new theater you see people telling them to run more cable and conduit then they think they will need. It's cheap to do when it's being built, but can become extremely expensive later down the road. You also wouldn't terminate it with the ends you linked to, you would use panel mount connectors that usually run more like $10 plus. No room in the current box at either end of the conduit? Going to have to drop another $20 a side for a box and cover to put that new data run in. Then maybe your AHJ is really strict and you have to pay an inspector to come in and inspect your work on top of all of that. A DMX Y cable to squeeze another universe out of an existing cable isn't a perfect solution, but it is another tool in the tool box that can be pulled out when your in a pinch. And as a theater technician you can never have enough of those IMO.
TuckerD
Well-Known Member
Ah. I had considered international markets. And you are right that it varies by situation. A good tech can make decisions like that based on his / the crew's labor costs. And cat5 utp might not be the best choice.
I know every venue is different. The perspective I was attacking this from was from the venues I have seen and worked in. I have never encountered an environment where the best decision if this situation had presented it's self there would be to add a Y over another cable. But I'm not a tech by trade, I am a computer programmer.
In the places I have worked with, adding another cable from the booth to the stage isn't as much of a problem as it would be in the spaces you mention.
And obviously you would want different connectors if this was going to be in the wall permanently. In that case then you shouldn't be building Y's Just hide the whole thing and hide the fact that you have two universes 1 cable by adding another box with two panel mount connectors.
This conversation gets really tricky when you start looking at very specific installations because the techs who work there know their venue best.
I know every venue is different. The perspective I was attacking this from was from the venues I have seen and worked in. I have never encountered an environment where the best decision if this situation had presented it's self there would be to add a Y over another cable. But I'm not a tech by trade, I am a computer programmer.
In the places I have worked with, adding another cable from the booth to the stage isn't as much of a problem as it would be in the spaces you mention.
And obviously you would want different connectors if this was going to be in the wall permanently. In that case then you shouldn't be building Y's Just hide the whole thing and hide the fact that you have two universes 1 cable by adding another box with two panel mount connectors.
This conversation gets really tricky when you start looking at very specific installations because the techs who work there know their venue best.
Other things can really add to the cost. The venue I do the dance recitals at is on the local historical registry, so we can't make any permanent changes without involving the town board. Another one has asbestos and lead paint, so cutting anything requires remediation; I finally had enough changes to do that I got approval and ran extra conduit. It cost $3000 for the asbestos removal and $400 for air quality testing, and that was for about 5 square feet.
Sure, if I had the option, I would pull new cable. I'd also much rather split a 5-pin or run temporary cat 5 before trying to run DMX up an audio snake. Even wireless has its place.
/mike
Sure, if I had the option, I would pull new cable. I'd also much rather split a 5-pin or run temporary cat 5 before trying to run DMX up an audio snake. Even wireless has its place.
/mike
RickR
Well-Known Member
You guys have expensive XLRs! Most of our suppliers sell Neutrik male or female, solder on, etc for $10USD-ish list! Online I see 5-8. Shipping is a pain but really, $27?
Anyway; in the few cases I've done this I considered pulling new wire, for maybe 15 seconds. Getting an electrician to show up is a minimum several hundred. 2-3 old Belden 9729 or similar in a conduit (ETC RFU + LINK + DMX) means pulling new wire is a full week project for 2. Remove old wire, pull new through several points, re-terminate, test, fix. In some places the conduit is full and the conduit is in poured concrete floors & walls.
Then there is the admin. It would cost several hundred for my time to get the estimates, convince the school district facilities folks, follow up with the district admin. Building permits!
Note that in this calculation the materials aren't even a significant percentage of the job. Well, you get the picture.
In some cases I went with replacing the wall jacks. That means actually doing work on the building but it's minimal enough. Frankly making the adapters is harder than it sounds. On one job I tried to make the obsolete RFU cable do the job. Its rated a for it and it should have worked, but it didn't, still don't know why.
Anyway; in the few cases I've done this I considered pulling new wire, for maybe 15 seconds. Getting an electrician to show up is a minimum several hundred. 2-3 old Belden 9729 or similar in a conduit (ETC RFU + LINK + DMX) means pulling new wire is a full week project for 2. Remove old wire, pull new through several points, re-terminate, test, fix. In some places the conduit is full and the conduit is in poured concrete floors & walls.
Then there is the admin. It would cost several hundred for my time to get the estimates, convince the school district facilities folks, follow up with the district admin. Building permits!
Note that in this calculation the materials aren't even a significant percentage of the job. Well, you get the picture.
In some cases I went with replacing the wall jacks. That means actually doing work on the building but it's minimal enough. Frankly making the adapters is harder than it sounds. On one job I tried to make the obsolete RFU cable do the job. Its rated a for it and it should have worked, but it didn't, still don't know why.
TuckerD
Well-Known Member
That was for four parts Rick. And like was determined. It depends on the situation, obviously in yours the cost is way higher to add a cable. At the venues I am most familiar with that wouldn't be they case. And in those situations, which I modeled, the cost difference is insignificant.
I do have another question though. How relevant is this problem, how many of you are actually in situations where you are using DMX like this rather than ACN?
I do have another question though. How relevant is this problem, how many of you are actually in situations where you are using DMX like this rather than ACN?
RickR
Well-Known Member
Most of these jobs have been public high schools, all designed by the same consultant with extremely similar systems, architecture, problems... Other places get other solutions.
If there was any possibility of installing a network for under $5000 I would have gone that way. But the calls were for A DMX output on stage, making it a second universe was a bonus! One place got 2 outputs because it was easy.
If there was any possibility of installing a network for under $5000 I would have gone that way. But the calls were for A DMX output on stage, making it a second universe was a bonus! One place got 2 outputs because it was easy.
Jay Ashworth
Well-Known Member
To clarify one point: inductive reactance and capacitive reactance combine with DC resistance to create AC impedance.
Cabling also generally has an interconductor capacitance spec which is (usually) in pF/ft; best I remember, that doesn't interact *directly* with the characteristic impedance of a cable design, since a 100ohm cable is a 100ohm cable, no matter how long it is. But I might be wrong; my Technician exam was 3 decades ago.
Cabling also generally has an interconductor capacitance spec which is (usually) in pF/ft; best I remember, that doesn't interact *directly* with the characteristic impedance of a cable design, since a 100ohm cable is a 100ohm cable, no matter how long it is. But I might be wrong; my Technician exam was 3 decades ago.
