DMX Cable - Are they telling me porkies?

[Ashley R]

As we are in the mood for talking DMX practice, can I point everyone to the excellent "Recommended Practice for DM512 - A guide for users and installers" 2nd Edition, by Adam Bennette and produced by PLASA/USITT. It's only a tenner and is really clear on the use of DMX and design of RDM compatible systems.

 

That seems like a good resorce to have handy.

 

After a bit of googleing is that that book? And any other/ cheaper places to get it? - http://www.plsnbookshelf.com/catalog/product_info.php?products_id=222 As im getting the feeling im going to be scared by the shipping price.

[indyld]

After a bit of googleing is that that book? And any other/ cheaper places to get it? - http://www.plsnbookshelf.com/catalog/product_info.php?products_id=222 As im getting the feeling im going to be scared by the shipping price.

 

I can't seem to find where you guys down under get hold of these publications, however, for readers in the UK the book is available from LSI Online Recommended Practice for DMX512 (2nd edition

 

As the BR has a good many members from different parts of the world, maybe someone will pop up with a more local source.

[Ashley R]

I Highly doubt a book like this would be avalible in Australia - the LSi price is much better, by AUD$10.00

PLSN - USD$32.00 (AUD$30.20) inc Shipping

LSi - GBP14.50(AUD$22.40) inc Shippping

 

Should be a good read when it gets here.

 

After reading the topic, it seems we have drifted quite a fair bit, from the original question

[Brian]

DMX cable IS lower quality than audio cable. It doesn't need to be as high, the integrity of the signal is not as dependent on it. Audio cables carry an analogue wave, DMX carries 1s and 0s.

Where to start with the above????

 

1/. The signal integrity absolutely does depend on the cable. After all, once you remove the cable there is nothing else to compromise the signal quality.

 

2/. Thinking of DMX as 1s and 0s is a mistake. It leads people to believe that the signal is solid voltage levels; maybe in the 5 Volt region. It's not. A typical driver, when working into a terminated and fully loaded line will exhibit a differential output swing of less that 2.5v when running on a 5V supply rail. When running on a 3.3V rail, a figure which is becoming increasing common, you may be looking at 1.5V or less.

 

This is fine because a receiver only needs a +/-200mV swing. However to achieve a quality signal with low error rates you will need to make sure the signal is not attenuated to much.

 

A DMX signal is an AC signal with a DC bias caused by the asymmetric waveform. Therefore all the rules on transmission lines apply. As well as DC resistance the Capacitance and too lesser extents the Inductance and Leakage resistance all come into play. That's why people use data cables.

 

A cable with too high a Capacitance will play havoc with your slew rates and eye height. Cable Capacitance is determined by the cable construction method (aka lay-up), the dimensions of each part and too a lesser extent the materials used.

 

The problem with DMX is that it has no error detection or correction. A one-bit error can cause all sorts of problems. If the bit in error is the MSB of a brightness value it can mean that you'll see a momentary flash. If it's the MSB of a position value then your head may well want to spin 180 degrees the other way before instantly coming back. The software in better quality heads will often smooth out such changes but this can introduce problems with how responsive a unit 'feels'.

 

To the system your cable looks like a low-pass filter (treble filter in audio speak).

 

To work properly your DMX signal needs to remain a close approximation to a square wave. Your cable will see that this doesn't happen. As the attenuation is frequency dependent each harmonic frequency needed to maintain the square wave is affected worse than it's predecessor.

 

Basically; there is a lot of published material out there from the people making DMX (RS-485) transceivers for people who think that cable doesn't matter.

[indyld]

Of course, while "old" 3 conductor cables can be used with RDM, splitters/buffers and any other gateways in the universe need to be compatible in order to let this traffic pass - so non-RDM buffers create a roadblock. However, compatible and non-compatible devices can be used in a configuration where the roadblock doesn't matter. But we lose the functionality of RDM after that block.

 

While not wishing to go too far down the DMX systems/RDM topic in this thread, useful though it is, I'm troubled by my description of RDM compatible splitters now I read it back and it's too late to edit the post.

 

The bit that might be misleading is the use of the word "roadblock" in an attempt to simplify things and enable a visualisation of a system of mixed equipment, RDM and non-RDM where RDM data can be used up to a point but from then on, it can't work. This could be misconstrued.

 

The reason I don't like the word I used is that, as RDM boffins will tell you, the problem with non-RDM splitters/amplifiers is that they are not correctly terminated and don't actually, er, block. The use of the RDM filter switch on a compatible DMX splitter does in fact create a roadblock to RDM traffic, stopping RDM from passing and potentially confusing the non compatible equipment, including subsequent non-RDM splitter further down the line.

 

I'm not sure if that more clear, or less, but I know that the BR smiles on accuracy and clarity and my use of the word was unfortunate, given the technicalities of how RDM systems actually work. Apologies.

[Ashley R]

Just a thank you to all that have posted.

 

My book(Recommended DMX Practice for DMX 512 2nd Edition) just turned up in the post yesterday, a very very good read, and has cleared up a lot of questions and doubts that I had in my mind. I would recommend this book to anyone.