DMX Terminator

[fincaman]

Hello All, this is my first post in lighting, I recently bought four led PAR lights each with 7 X 4 way LEDs (RGBW) . When I put the terminator in the last fixture it flickers but works fine without it. I was using a 10 meter normal mic lead (not DMX cable) from the controller. It doesn't bother me because everything works fine without the terminator. I am just puzzled !!!

[dbuckley]

What this means is there is something wrong with either your equipment or more probably the wiring. A correctly placed terminator will never make matters worse on a correctly configured system.

[timsabre]

What this means is there is something wrong with either your equipment or more probably the wiring. A correctly placed terminator will never make matters worse on a correctly configured system.

 

Yes, normally it means there is a break in either the hot or cold DMX wire somewhere along the line.

Start with one fixture, one cable and the terminator, then add further fixtures until the fault appears.

[marktownend]

And be aware that the fault could be intermittent - I had a similar issue where the fault turned out to be bad connections inside one (or possibly more?!) of the fixtures where the XLR connectors for the DMX were soldered directly onto a PCB, meaning that as I was changing cables around the fault was not appearing and disappearing logically.

[Jivemaster]

On economy DMX units they do sometimes not read the signal correctly -sadly it's just a fact of cheap kit. I had some colour changers once that ONLY worked in one order on the DMX cable.

[fincaman]

Hello again, and thanks for trying to help, I am going to set it up as I did on the gig and see if I can reproduce the fault, then report back. Watch This Space !!!

[boatman]

Some cheap LED fixtures require the full universe of 512 channels of DMX to be sent to make them work reliably. Try setting up your controller that way and see if it helps.

 

Edited: to remove unnecessary quote.

[fincaman]

Hello Again, I have been experimenting since I last posted and found that whatever combination of lights and cables I used I always got flickering on one or more Pars with the terminator in and it always worked perfectly with the terminator unplugged. I looked inside the terminator which I made 120 Ohm resister between pins 2 & 3 correct. I got out my computer loaded DMX Control and plugged in my Velleman USB to DMX interface (which I made) and got flawless results whether the terminator was in or out so it looks as though it's my DMC controller which is a cheapo Stairville DMC Master that is at fault.

[bigclive]

There can be massive variation in the quality of DMX output circuitry between desks. Some use proper drivers and some cheap desks fudge it. The two most common issues "caused" by terminators are revealing a wiring fault in the run or being the straw that broke the camels back in a run that is pushing the desks output circuitry to its limit. This can be aggravated by equipment that uses opto-isolators as DMX receivers, since they pose a higher load than a proper receiver and need a higher voltage to detect data reliably than a proper receiver.

 

I'm pro-terminators, but I know so many seasoned veterans in the industry that only use them as a last resort because they've had a bad experience in the past where a stable rig had issues with the terminator in. Usually down to bad connections or damaged cables.

[Kazeja]

To test for wiring issues in a DMX chain (no buffers etc) with everything turned off, measure the resistance between DMX+ and DMX- at the DMX source. With a terminator fitted and allowing for component tolerances, you should be seeing something in the region of 100 to 130 ohms. Anything in the Kilohm range would indicate a broken wire in one of the legs. Anything in 60 ohms or less range would indicate over termination or a low resistance short on the line.

 

Please bear in mind that ALL connected DMX equipment in the chain MUST BE TURNED OFF or you may experience strange readings depending on the meter you're using.

 

K

[fincaman]

Thanks all for helping, all my cables have been checked and no fault found but guess what ? after an hour or so of running the PARs on my Velleman - computer setup the lights became erratic again I removed the terminator and they worked fine and as I have used the same setup with my other led PARs it must be the new ones that are at fault

[timsabre]

Thanks all for helping, all my cables have been checked and no fault found but guess what ? after an hour or so of running the PARs on my Velleman - computer setup the lights became erratic again I removed the terminator and they worked fine and as I have used the same setup with my other led PARs it must be the new ones that are at fault

 

It sounds like the lights are putting a larger than usual load on the DMX line.

[Showbitz]

In my experience a terminator is rarely required and is only really useful if there is an issue.

 

Also some of the cheaper lighting desks don't output the full 512 DMX signal, rather, if they are 48 channels, they sometimes output the first 48 channels of the DMX then ignore the rest meaning that rather than a continuous stream of data down the DMX line it effectively pulses, this can cause the cheaper LED fixtures to become confused and flicker too. - I have some cheap LED pars, they will work fine on a Strand 520i but will flicker on a Showtec Showmaster 48 for exactly this reason.

 

Also the use of a mic cable probably isn't helping matters...

 

Hope that is of interest

[alistermorton]

In my experience a terminator is rarely required and is only really useful if there is an issue.

The longer the cable and the more devices, the more you're likely to need a terminator, IME, although you can often get lucky.

 

Also some of the cheaper lighting desks don't output the full 512 DMX signal, rather, if they are 48 channels, they sometimes output the first 48 channels of the DMX then ignore the rest

Which is perfectly valid within the protocol definition, of course.

[bigclive]

Some cheap lights may rely on the full DMX packet length to grab their little bit of data and then spend the rest of the time processing it. If they are getting a short packet (which is perfectly acceptable as mentioned) then they may be getting their processing cut short or missing packets because they were too busy to look for them.