DMX Terminators

[Michael Beere]

Just a Quick question, If you have a DMX line with 10 movers on it and right on the end of that line you have 2 LED Pars, why do the LED Pars decide to slightly flash in and out when sat in a cue when not programmed in? Is this because I don't have a terminator in the last device?

 

Can anyone help?

 

Cheers

 

Mike Beere

[P. Funk]

Somethign like that - sounds like dodgy data at least. Are you using proper DMX cable, or mic cable? Does it still happen if you plug the DMX straight into the LED pars?

[Ben Langfeld]

Could be for a multitude of reasons. Try going through them in this order:

1) Terminate the line. There's no reason not to. It's just good practice. Always terminate DMX lines.

2) Check the cans aren't set to sound-to-light.

3) Monitor the DMX levels sent by the desk for the channels the cans are set to to ensure it's not the desk spitting out the wrong data.

[frazer]

It is always good practice to put a terminator in the last device anyway. Some people unfortunately dont think terminating DMX is always necessary but it does make things a lot easier when troubleshooting twitchy fixtures.

 

I've only just finished a pantomime in which I didnt (at first) have the end dimmer pack terminated. Most of the show it worked fine, but without warning the movers would start twitching and the lights would start flashing about randomly.

 

Do the PARs work correctly the rest of the time, I.e when being used in a particular cue? If they do, then you probably have answered your own question.

 

Hope that helps

 

Frazer

[Michael Beere]

Some answers to all those question. The desk wasn't given off any data when it shouldn't. When the cans where in cues they didn't flash once in the wrong place. The cable I was using was all 5 pin DMX cable with correct 3-5 and 5-3 jumpers. The cans weren't set to sound to light. I think it might be either it needs a terminator or don't run LED Pars on the same line as movers.

 

Cheers

 

Mike Beere

[Ben Langfeld]

I think it might be either it needs a terminator or don't run LED Pars on the same line as movers.

 

There's no reason why you shouldn't run LED Pars and movers on the same line. If terminating doesn't solve the problem, I'd be suspecting either the desk or the cans themselves. You could try running the cans straight out of the desk (as in locally to the desk with a short DMX cable) to rule out the desk and/or the cans as being faulty.

[Michael Beere]

L'll give that a try, Cheers for all the help

 

Mike Beere

[Tomo]

It sounds very likely to be a data reflection issue.

 

Terminate the line.

 

In simple terms:

The data signal travels along the line through all the fixtures, hits the open end and reflects back towards the lighting desk.

It then hits the data coming up the line, and everything gets confused.

 

The termination resistor absorbs the data signal, thus preventing reflections.

 

- In reality it's a bit more complex than this, but you get the basic idea.

[Dave Hallett]

I would be surprised if this was due to the data reflections that terminators are designed to eliminate. The reflections caused by an unterminated line will not cause problems at the end of the line but may confuse lights further back down the line towards the desk / buffer.

 

If at all possible I would certainly try to plug the LED fixtures directly into the desk to see if the problem persists. It may be that you have a dodgy light, cable or 5-3 adaptor in the system which has dropped a data leg and is weakening the signal. If adding a terminator makes the problem worse or kills the system completely then that's almost certainly the case. Leave the terminator in and try bypassing the DMX around 1 fixture at a time to narrow down the fault.

 

Cheers,

 

Dave

 

P.S. If you understand the theory of why terminators are required then you can often do without them. If you don't then just use them!

[dave singleton]

If your talking about the odd ocational flicker, they do that. Its most annoying. It doesnt matter how long the DMX chain etc they have a little flicker every now and again. Even one unit direct into the desk does it on mine.

[Don Allen]

Most cheap led products use switch mode power supplies which can generate interference back along the mains and also into the DMX lines. To see if this is a cause of problems on DMX lines, you need to use a DMX splitter for the suspect led product, preferably located close to the suspect device. This interference tends to be continuous, not intermittent.

 

A source of intermittent flashes can be due to the refresh rate of the suspect devices DMX receiver. There is a wide range of desk DMX transmit refresh rates and device DMX receive rates. In the better quality desks you can change your DMX packet rates. With more basic desks try substitution.

 

I know a DMX tester is expensive, I bought the Showtec one which when it boots up says Dr DMX 2 so I had a win there, but I can see the Tx and Rx DMX specs which takes a lot of the guess work out of troubleshooting DMX faults. In my DMX tester I have 3 to 5 adaptors and terminators with led's.

[MarcT]

Some LED pars have a macro or mode channel, which causes them to do some built in function. It may be that the "intensity" on the desk is off, but one of the other attribute channels is up causing it to do its own thing. Easily done if its a Strand desk you're using. Ensure the desk is sending 0's for all the LED can's channels.

 

It could also be dodgy cabling, or lack of termination.

 

A quick & dirty test is to fit a terminator on the last fixture, then using a multimeter set to the resistance (ohm) setting measure across pins 2 & 3 of the DMX plug at the start of the chain (the one that would plug into the desk). You should see 120ohms or there abouts. If so great - the cabling is most likely ok.

If its open circuit (or worse, short circuit), then work back from the end of the line until you find the fault.

 

The reason I say this is because I had an LED can which didn't have continuity between DMX in and DMX out on one pin. More recently I had the same issue on a hired Robe Colourspot 1200, causing everything else on the same line to freak out!

 

Oh, and ditto the above re DMX refresh rate. It could also be that your desk isn't spitting out the DMX frames quickly enough (or maybe too quickly). If you can change the DMX rate, try that - if not maybe try another make/model of desk.

 

Best Regards,

Marc

[Kelite]

I think Marc is on to something.

 

<Oh, and ditto the above re DMX refresh rate. It could also be that your desk isn't spitting out the DMX frames quickly enough (or maybe too quickly). If you can change the DMX rate, try that - if not maybe try another make/model of desk.>

 

The LED Can controlled from the desk all by itself should give you perfect performance. This very well may be a refresh rate issue. Can you slow down/ speed up the rate from the console?

[David A]

Some cheaper movers transmit DMX to slave other units I had this problem recently, one unit was transmitting data when it shouldn't.I found the problem by isolating one unit at a time until I found the culprit.With this much gear you should seriously consider a proper DMX tester or you waste a lot of time searching to find the problem

[ChrisD]

A problem I once found with an LED DMX unit (a cheapo disco one) was that it would randomly flash or spit out nonsense data from time to time. It turned out that it was fine running off a Strand 300, but when on a cheaper desk, it would play up. The solution for me (on my USB DMX box) was to create a fixture in the software with an address of 512 and a random intensity. It forced the software to output a full universe of DMX, and stopped the problem. If you're using a cheap desk to control the fixtures, it might be that the more advanced movers can cope with only receiving part of a universe, but the LED stuff wants the whole thing. Try them with a full universe, or on a better desk.