DMX STUTTERING

[djsteviec]

I work in a bar where they have moved the dj box from the fitst floor down to the ground floor. Its about 30 metres (cable length) away. I have just run a cable from the new box to the old for the controller (q commander) it then plugs into a DMX booster/splitter in the old box and fires off to a- the main lighting righ (4 robe moving heads, 6 robe colour spots, 4 martin mx-4 and 2 martin wizards) b- to 4 sets of LED strips and c- to a smoke machine that is no longer used.

 

All seems ok but the lights seem to flutter around, moving slightly now and again, or shaking position, or strobing on and off. Only slightly, but I know its not right.

 

Someone suggested ghosting and that I should put a DMX Terminator on. Any Ideas???

[Dmills]

All the lines should be terminated (including the one feeding the splitter).

You did use the right cable?

 

If everything is doing it, I would be looking at the termination of the line feeding the splitter, and possibly at the cable type. Earth voltage differences might also be worth investigation if the wiring is anything like all too many clubs.

 

Regards, Dan.

[djsteviec]

I was told that this cable would be ok.

 

http://www.theelectronicsshop.co.uk/cable%20screened.htm

 

How would I terminate the line going into the splitter? I thought the terminators were resistors across pins 2 and 3 in an XLR plug that goes in the last light in a chain. Am I wrong?

[Dmills]

I don't see anything there that I would consider to be reliably suitable.

 

What you want is ideally 120 ohm RS-485 cable, but AES Digital audio cable or even cat 5 FTP will do (use one pair). What you don't want is ordinary microphone cable (it may sometimes work, but...).

Each DMX chain should be terminated at the end remote from the transmitter with a resistor of 100 -120 ohms across pins 2 and 3. The splitter (depending on how it is set up) may or may not be the last device in a chain.

 

Regards, Dan.

[Jivemaster]

That looks like microphone cable! It may or may not work properly. The splitter should terminate the lead feeding it. Lines aa, b and c should be terminated with as you say a 120R between pins 2 and 3 at the end, though some devices will terminate internally either automatically or by dip switch.

 

Are the connectors up to standard? I've had DMX disappear completely at "economy" connectors.

 

Sadly this is typical of a poorly modded DMX line failing to function stably. This is usually blamed on using mic cable instead of DMX cable.

[djsteviec]

Unfortunately everything I did was budget. I only installed the 30m length and 2 XLRs, im not sure of the quality of the rest.

 

ill take the desk back up to the old spot where the splitter is and see if its got a problem when its up there. If it hasnt then ill know its the cable I installed.

[dirkenstein]

Oh well, everybody else beat me to it...

 

I was told that this cable would be ok.

 

http://www.theelectronicsshop.co.uk/cable%20screened.htm

 

How would I terminate the line going into the splitter? I thought the terminators were resistors across pins 2 and 3 in an XLR plug that goes in the last light in a chain. Am I wrong?

No, you're not wrong, leaving aside any discussions about 5-pin vs. 3-pin DMX connectors. The resistor should be 120 ohm.

A lot of splitters have built-in termination, which may be switchable. Otherwise one of the outputs will be designated as a through-output (physically connected directly to the DMX input without the signal being regenerated), which would be the place to plug your terminator.

If that output is in use you'll want to make sure the end of that chain is terminated.

 

The link you provided lists a number of standard two-core microphone cables. Although any of them might work for transmitting DMX over short distances, none of them are likely to do so reliably or repeatably in every installation. None of the cable types lists digital-grade transmission capability, characteristic impedance or loss/m at various frequencies. Only twisted-pair cable with 120 ohm characteristic impedance and suitable for RS485 high speed data transmission is really suitable for DMX- remember the clock frequency of DMX is something like 12x the highest frequency a mic cable needs to transmit reliably.

 

The cheapest, most widely available install cable that fulfills these requirements is Cat 5 network cable. There are a range of industrial rs485 cables available from Belden, and some cables for digital audio transmission (AES/EBU) may also be suitable.

 

You may be able to get away with using a microphone cable to hook up our DMX chain in an emergency. In fact, you might get away with using it exclusively it in 95 or 97% of events or installs you perform.

 

However, incorrect cable grade or missing/ improper termination will show up as precisely the sort of erratic problem you've described under completely unpredictable circumstances.

 

There's a lot of folk wisdom out there saying that cables aren't critical and that termination causes more problems than it solves. I have heard posters on this site arguing this position vehemently, backed by their own anecdotal and no doubt non-fictitious experience.

 

Feel free to read reviews of the cabling infrastructure for the Brit Awards, Glastonbury main stage or any other hight-profile event (where there might be several hundred universes of DMX in use at once), to find out to what degree the industry professionals are willing to trust folk wisdom.

 

It is, however always worth checking for cable shorts and breaks in one leg of your DMX chain- the signal will sometimes partially survive quite surprising degrees of cable damage and degradation.

[RobOwen]

ill take the desk back up to the old spot where the splitter is and see if its got a problem when its up there. If it hasnt then ill know its the cable I installed.

 

Not as simple as that. While moving the desk back may well make the link work again, it doesn't mean it's the cable as such. Your whole system might have worked by luck with no termination, but adding that bit of extra length has pushed it over the edge by slightly changing the lines resistance. The extra length to the splitter might be causing a reflection problem (or making them noticeable), and terminating the line will most likely solve it IF your connections are sound in the XLRs.

[boatman]

A termination should be fitted to all DMX cable runs. There have been several topics recently from posters who say that a termination causes their DMX installation to stop working. However that has inevitably turned out to be an undiscovered fault in the system somewhere. If DMX does not work when the termination is present then there is a problem with the cabling and it should be investigated.

[indyld]

There have been several topics recently from posters who say that a termination causes their DMX installation to stop working. However that has inevitably turned out to be an undiscovered fault in the system somewhere.

 

Agreed. This is a common mistake when fault finding DMX. As has been said, sometimes that last bit of cable is enough straw to break the camels back when the problem is actually further upstream.

 

A DMX buffer in the system can also mask cable faults that otherwise would show up.

 

Here is a quick and dirty article on fault finding DMX problems for anyone who is interested.