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DMX512 Tips

Using a daisy chained data network like the DMX512 is not without it's associated problems but experience has shown that , more often than not, it's bad cabling and bad electrical installation which is responsible for all pre-gig headaches.

Here are a few tips you can follow while rigging and testing:

1) First and foremost, get yourself a hand-held DMX512 test unit if you don't have one already. This is absolutely essential. It will help you to detect errors all around the rig and is most useful. There a number of companies offering different models.

2) Always use proper cables. Microphone balanced line cables are NOT suitable for DMX512 work. BELDEN 8227 is an cable I can recommend since I have used it but there are lots of other types.
You can find a selection of suitable brands in a chart at the bottom of the PHYSICALS page.

3) Check each cable for continuity and phase before it get's rigged up. Pins 2 & 3 should not get reversed BUT in some cases it NEEDS to be REVERSED.

4) Be careful of receivers using 3 pin DMX512. Sometimes you may HAVE TO use 5 pin XLR to 3 pin XLR conversion cables WITH THE PHASE REVERSED. I have faced this problem and I know the pain. It's always safe to know in advance what the rental company is going to give you and then look up the unit's USER'S MANUAL to check on connections. It's better to carry a fine tipped soldering gun for on the spot reversing of cores.

5) Check the earthing scheme of each receiver. PIN 1 of the DMX input or output should NOT be connected to the unit's chassis. Use a continuity tester or multimeter to check wether there is continuity between PIN 1 and the chassis. If the tester beeps or shows continuity, you must use a DMX isolator between that unit and the rest of the chain. Only the console should have it's output ground connected to the chassis.

6) Some VGA or SVGA monitors tend to have mains leakage on their signal connecter. Always switch on an unknown moniter separately BEFORE connecting to your console and check the moniter's connecter with a mains tester. I once got a nasty jerk way down the DMX chain.....only to find later that the terminal had leaked almost 3/4 's of the mains potential into the DMX output of the console. In any case , check and see that the console's chassis is properly grounded.

7) Connect all intelligent lights to a seperate mains which is isolated from the dimmer racks or the audio power amps. A glorious burst of 200 parcans looks good on stage, but can happily reset all the intelligent fixtures instantly if the power cabling is overloaded. The same goes for a single bass guitar note in a rock concert at 100Kwatts . This HAS happened !!

8) Loose fitting XLR connectors can always be nagging problem. Use good connectors always without compromise. Loose connections can be often solved by GENTLY bending OUT the MALE connector 's pins. This is a LAST RESORT only.

9) REMEMBER to insert a 120 ohms termination plug/plugs in the LAST unit of the chain/chains. Have at least 5 of each type - 5 pin male and 3 pin male ( Oh yes , you can land up with a 3 pin unit at the fag end of a chain ). Always test the plugs before they go up into the rig or you will have an reluctant rigger (no offence, guys !!) going up again to fix a lowly plug !! Check and see if the last unit has built-in termination facility (usually a dip switch to be flicked) in it and activate it ,if it does.

10) Calculate your DMX loading and use line drivers/splitters if needed. Even though the code says you can connect 32 units on a single DMX512 leg, I would recommend, from the practical standpoint, not more than half that number. Plan your DMX512 legs manufacturer wise. Use a DMX512 splitter and put units belonging to each manufacturer on each splitter leg ending with a terminator. This approach has solved many problems for me in the past. Put a different manufacturer's unit on the same leg only if it is geographical solution to a cable run problem.

11) Plan your address setting carefully. Whenever possible, allow your console to generate your fixture/dimmer addresses and always take out a printout if your console has the facility. If you need to manually generate address, do so with great care considering that it is very easy to overlap channels while calculating the patch. After rechecking you addresses, generate the neccesary dip switch according to the fixture concered. Some fixtures use a different format dip switch settings. Some dip switches also set other parameters such as PAN & TILT swap or PAN INVERSE or TILT INVERSE. When similar fixtures are at opposite ends of a rig and facing one another you may need to invert the PAN on any one of them.

12) Avoid running DMX lines parallel to power cables and dimmer load cables as much as possible. Running the cable alongside a audio console snake is OK.

13) Be careful when routing DMX lines near luminaires like PARcans, exhaust fan outlets in intelligent lights ,etc. They can toast up your line before the 1st act. Use heat resistant cables with polyurethane jackets and avoid direct contact.

14) Using an ON-LINE UPS (Uninterrupted Power Supply) to feed power to your console is always recommended. The ON-LINE UPS is not so much for retaining power in case of a shut down, as it is for avoiding spike and brown-outs. An OFF-LINE UPS is NOT recommended. Retaining proper data flow from any microproccessor is ensured by a stable power supply. A brown-out or spike will not cripple a good proccessor but can cause it to vomit bad data for a fraction of a second.

12) Once again, read the user manuals of unfamiliar intelligent units like scanners carefully before using. Or look up the DMX usage data for the unit from the manufacturer 's website or the ESTA DMX USAGE PAGE.
Your console may have a library that assigns the parameters correctly to respective data-wheels BUT raising the INTENSITY fader MAY NOT allways bring up the beam ! Some times the STROBE value needs to be 100% or 255 or &hFF for the beam to work...and so on ! Unfamiliarity will lead to unneccesary plotting delays and no 'break' before the event starts !!

13) Preferably, use an optoisolated splitter leg, as a rule, for dimmer racks. I, once, had a blown DMX chip (SN75176) in a console, because of someones's faulty dimmer !

Like I said in the beginning, I'll really keep this page up-to-date with a little help from everybody !!

Copyright: Ujjal Kar | ujjal@standardrobotics.com

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