DMX splitter

Mark

I'm going to ask the question not yet asked

Why do you see a requirement for a DMX Splitter?

The answer to this question could be useful in providing good recommendations.

You mention RGB but not the controllers etc planned to hang off the DMX cable. It is actually quite unusually to reach a device limit in this hobby with the use of 16ch+ controllers.
If you are however using a lot of those cheap little 3ch chinese controllers then a splitter or repeater becomes a requirement.
If you have a lot of the 3ch type units then a good quality splitter preferably with optical isolation would be a good idea, this will reduce the likelyhood of unintentional ground loops from multiple different power supplies on the DMX common wire.

Cheers
Phil

 

Gents

Everyone has good points but we are getting off track

I have more to say but only on phone atm

Phil

 

Ok got a few minutes to expand on my reply......

Standards! well if people are going to open the door then please get it absolutely correct.

Concerns:
Greg, we may rarely agree but in this case i agree that the use of y-splitters is a bad idea even if it works, your discussion on this would have come across better if it didn't read as a marketing exercise.

David (Dmoore) if your happy to support your customers when they use those splitters as you have had them work then i'm not going to say don't post it as an option but it should come with the caveat that it is not the way EIA-485 Physical Layer should be cabled.

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Now to Standards
EIA-485 - From Wikipedia http://en.wikipedia.org/wiki/RS-485
EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the EIA-485 standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multi-drop configuration

Take note of the word "linear" in the above quote, the physical media standard does not allow "star" cabling configurations.

DMX512/DMX512A - Again from Wikipedia. http://en.wikipedia.org/wiki/DMX512
DMX512 is a standard for digital communication networks that are commonly used to control stage lighting and effects. It was originally intended as a standardized method for controlling light dimmers, which, prior to DMX512, had employed various incompatible proprietary protocols. However, it soon became the primary method for linking not only controllers and dimmers, but also more advanced fixtures and special effects devices such as fog machines and moving lights, and has also expanded to uses in non-theatrical interior and architectural lighting; DMX512 has been used at scales ranging from strings of Christmas lights to electronic billboards.
DMX512 employs EIA-485 differential signaling at its physical layer, in conjunction with a variable-size, packet based communication protocol. It is unidirectional.
In 1998 the Entertainment Services and Technology Association (ESTA) began a revision process to develop the standard as an ANSI standard. The resulting revised standard, known officially as "Entertainment Technology—USITT DMX512-A—Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories", was approved by the American National Standards Institute (ANSI) in November 2004. It was revised again in 2008, and is the current standard known as "E1.11 - 2008, USITT DMX512-A", or just "DMX512-A".
As of January 2011 ESTA, the Entertainment Services and Technology Association in North America, has merged with PLASA a similar organization in the UK. The new organization is called Plasa. [2] All ESTA standards are now maintained by Plasa.

POE or more specifically Passive POE.
Again from Wikipedia - http://en.wikipedia.org/wiki/Power_over_Ethernet#Passive
Numerous devices exist which make use of positive power received on pins 4 and 5 of the Ethernet cable, with negative return on pins 7 and 8. Power over Ethernet injectors exist which convert 110-240 V 50/60 Hz AC power to 9 V, 15 V, 24 V, 48 V or 55 V DC with currents ranging from 0.5 A to 2.0 A. In the common "passive" PoE system the injector does not communicate with the powered device to negotiate its wattage requirements, but merely supplies power.

Now interestingly Passive POE and P-DMX are the same thing from a POWER perspective, both utilise 4-5, 7-8 and guess what that is actually compliant with the 802.3af Mode B standards for physically media.

The term pDMX may be a poor choice when these sorts of discussions occur.

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Now personal comment: (as i need to check the actual E1.11 standard that i own)
E1.11 does not explicitly deny the use of Passive POE but does make reference to the use of 4-5, 7-8 as P-POE wiring when using 8 wire twisted pair cable.
What however E1.11 does say is that the use of Cat5 type cable is limited to "fixed" installation.

NONE of that matters though at the protocol level, if the sender and receiver are sending communications that have the DMX Frame structure then they are using the "DMX" protocol.
What matters is that unless the device actually meets ALL of the standard it cannot be labeled as DMX512 or DMX512A.

Thinking through all the items used in the Community across all the forums it is a rare device that could be labeled with DMX512/512A

E1.11 also does discuss exceptions to the EI-485 standard for physical media grounding.

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How does all this relate..... it probably doesn't much but the standards door was opened so I walked through it.

At a practical level the use of Passive Y type splitter is not supported or recommended for use by professionals in the field but may actually work as Dmoore demonstrates.
Only Active type splitters should be used when looking to implement a setup that is as trouble free as possible.

Regards
Phil

 

One additional note, David_AVD pointed out to me that one of the uses for those Dual RJ45 adapters is to provide a drop from the linear cabling to the physical controller where the controller only has a single connection point.

This is valid under the standards only when the Stub length (the cable to the controller) is keep really short.

Define really short? -> i've seen figures from a few inches to multiple feet so unless someone can point to a definate distance listed in a standard i'm going to say the rule of thumb is less than a foot.

However: this document gives evidence that it can be much longer, note the measurements are specific to the particular RS485 IC's used.

http://www.ti.com/lit/an/slla272b/slla272b.pdf


Cheers
Phil