RGB, DMX, LOR, Cat cable and power injection

[Ponddude]

To forget about the ground, no matter if it is in the holiday lighting industry or some commercial environment is just bad practice. Steve is 100% correct that a ground is needed. In order for DMX devices to fit within the DMX spec it must have a ground line. Not only for communication but safety also comes to mind.

Another extremely poor thing to do within any RS485 network is just split a data line with cheap phone splitters. There is not a single day that goes by that I don't get an email or phone call from a customer asking about a "fix" for their data transmission issues. The reason is, without question, splitting a data line is 100% against the DMX standard established by USITT. If a data line is split (I am specifically speaking of DMX data) it MUST be repeated, multiplied or buffered. Degredation and signal loss is guaranteed to happen with in "split" networks and you need to buffer that data somewhere along the line. Will it work, sure, but the more data you try and send down a non-buffered (just made that word up!) data line the more trouble you can expect.

Anything against the standard can not, and will not, be considered DMX...it is more of a made up protocol.

Greg

[Steven]

dmoore wrote:

If I was building a factory using Modbus over RS-485, heck yea I’d do a ground. For Christmas lights, I’d personally rather use that wire for power carrying functions.

You can still use the ground for power carrying functions. The cheap 3-channel DMX controllers do this: The ground wire on the DMX cable goes to the same trace as the ground on the power input cable.

I am suggesting that you put the negative lead of the power supply on pin 7, rather than the positive. You can still use 3 pins for positive and 3 for negative.

[Timon]

Let me through my $0.02 in on this issue.

You are totally correct about the grounds but not for the reason you think.

RS485 only needs 2 wires to function. Ground is not a requirement except to keep each end at close to the same ground potential else there will be too much common mode for the chips to function correctly.

So to maintain the specifications one should always have a ground connection between devices which means on RJ45 connectors you MUST have 7 and 8 grounded.

HOWEVER, if the board has a RS485 section that is totally isolated from the any and all ground connections,many of the AVD controllers out of Australia are made this way, then you can run without any ground connection and still be following the RS485 specification.

BTW, this whole ground issue will mainly be an issue when connection over very large runs which are powered from two different power feeds such as you would see when connecting to your neighbors house across the street. If your powering everything from your own house the ground differences will likely not get high enough to cause problems. That said, don't depend on it especially in older homes built before our current wiring standards.

This is why I support only doing power over RJ45 DMX using the P-DMX standard which differers from the ESTA Standard only by using the unused pairs, 4 & 5, for power. I then make sure I don't go over about 1.5A total.

Hope this helps.

[Steven]

Ponddude wrote:

To forget about the ground, no matter if it is in the holiday lighting industry or some commercial environment is just bad practice. Steve is 100% correct that a ground is needed. In order for DMX devices to fit within the DMX spec it must have a ground line. Not only for communication but safety also comes to mind.

So are you going to change the Blender and MultiPort Blender so that pin 7 of the DMX input goes to ground?

[Timon]

Steven wrote:

dmoore wrote:

If I was building a factory using Modbus over RS-485, heck yea I’d do a ground.  For Christmas lights, I’d personally rather use that wire for power carrying functions.

You can still use the ground for power carrying functions.  The cheap 3-channel DMX controllers do this:  The ground wire on the DMX cable goes to the same trace as the ground on the power input cable.

I am suggesting that you put the negative lead of the power supply on pin 7, rather than the positive.  You can still use 3 pins for positive and 3 for negative.

Note on all DMX controllers that follow the ESTA Standard both pins 7 & 8 are grounded. Putting positive voltage on either is a BAD idea.

[Ponddude]

There are not any plans currently because as you pointed out the standard states that the RS485 chip CAN be grounded to the board. USITT states this directly in the protocol documentation.

I can speak to the designers about adding it though.

Greg

[rmturner54]

Just remember that this is a hobby. SO as far as power injection, which is what I believe this thread was all about, there is no standard. I personally like the injection concept for certain elements. As far as cable. The only deal breaker for me was waterproofing the cat5 connectors. They do make a waterproof boot but is not cost effective for me. If I were to use cat5, it would have been stranded. So I used 4 cond speaker cable for DMX and power.

[spomalley]

The only deal breaker for me was waterproofing the cat5 connectors. They do make a waterproof boot but is not cost effective for me. If I were to use cat5, it would have been stranded. So I used 4 cond speaker cable for DMX and power.

Many people have asked for advice on how to waterproof CAT5 jacks, couplers, ect and the best answer that we've received is to spray everything with CorrosionX.

[aussiephil]

Just remember that this is a hobby. SO as far as power injection, which is what I believe this thread was all about, there is no standard. I personally like the injection concept for certain elements. As far as cable. The only deal breaker for me was waterproofing the cat5 connectors. They do make a waterproof boot but is not cost effective for me. If I were to use cat5, it would have been stranded. So I used 4 cond speaker cable for DMX and power.

Respectfully i have to disagree with this, for people using CAT5 cables MOST of the hardware designers have gone the distance an use the published standards.

The P-DMX wiring is pure 802.3af 10/100 ModeB Standard as is PassivePOE

Controllers in the community using P-DMX/PassivePOE (they are the same) have all used the same wiring and expect DC voltage from 9-36V.

All the same as major manufacturers that have/do use PassivePOE

Even LOR at least has been completely consistent and it is dead easy to make the adapter cables.

Using 3 pairs is not power injection, it's abuse of the Cat5!

Cheers

Phil

[Ponddude]

No one ever wants to listen to me

Greg