Testing dmx cable

[unc417]

I've just ordered a SM Pro Audio CT-3 cable tester, and it tests 5 pin DMX. But we use all 3 pin DMX at my facility. For testing purposes, can I use the 3 pin XLR on the tester or do I need to get DMX adapters and use the DMX jack?

[daver2]

You should be able to use the 3-pin XLR test plug and socket to test 3-pin DMX cables.

There should be no need for any 3-5 pin adapters.

Dave

[unc417]

Great. Thanks

Anybody have any experience with the ct-3? Is there another tester you can recommend? I couldn't find many testers that had a variety of connections and was in 2 pieces.

[jimmol]

Welcome to the forum,

What exactly are you trying to find out about your CT3?

For testing your DMX 3 pin your CT3 should work fine. It has the 3 pin XLR tester.

Most testers tell you if the cables and pins are connecting. The "hot", "cold", "ground" and shielding are good.

I see the CT3 sends a signal on all pins and the other half reads it. The manual even does a good job of telling you what to expect lights to expect. The manual is better than what I received with my Behringer tester, but still not the best for the person just starting to learn how to test.

If you are trying to check the impedance, to see if it indeed a 120 ohm cable, you probably need a multi meter. I have not done this, so someone else will need to check in on how to if that is what you wish.

They do make 5 to 3 pin adapters, but since the CT3 does not test impedance on the 5 pin connecter, there is no reason for it, just use the 3 pin xlr.

I will have to look into getting the CT3 as it looks great for checking snakes.

[unc417]

I bought the tester because I think I'm having some dmx issues. Recently some of my led fixtures started randomly strobing when no such command was activated from the dmx controller (pc based: Enlighten). I do have a behringer xlr tester, but being mostly an audio guy and new to lighting, I didn't even think about the possibility of using an xlr tester for dmx cable. I just assumed there was a difference in the xlr and dmx testing functions. I would have bought the ct-3 anyway though, it looks to be a great tester.

[daver2]

A cable and a connector is a cable and a connector as far as the cable tester is concerned. It doesn't matter if the cable is used for audio or lighting. The cable is used to link pins of the connectors at either end together. The important thing is that the correct pins are connected together (i.e. the cable is free from short circuits and open circuits).

Problems with DMX lighting fixtures usually stems from people installing microphone (or other non-digital) cable rather than proper digital DMX cable. The other gotcha is that at the end of the DMX run should be a 120 ohm terminating resistor.

So, first check out what type of cable has been used for the install. There should be some markings on the outside sheath of the cable. Second, check for the presence of the terminating resistor on the last fixture of the DMX chain.

Dave

[unc417]

All of my dmx cable is custom made by my local a/v dealer. There is no terminator on the last fixture. I wasn't aware a terminator was needed? The last fixture in the chain is a Chauvet Colordash Quad.

[RickR]

Look at www.dfd.com & www.goddarddesign.com These companies make testers that are far more than a circuit continuity check.

One of the key steps in troubleshooting DMX is dividing the system into parts, so you can figure out which component is causing it all. Testers make it far easier but some things you can do without them.

• Take your console out to various places in the rig and control small parts by themselves.
• Bypass individual fixtures by plugging the input and output cables together.
• Bring individual fixtures to the console and use a very short (known good) cable.
• Use a long cable to bypass permanent wiring, dimmer racks, splitters and such.
• Add and remove terminators.

For each of these tests look for improvements or other changes. Most times you can isolate a single device. A cable can often be repaired if it's the end. Fixtures usually require some expertise and parts.

[Peter Coleman]

If you can get your hands quickly on a DMX terminator, then that may solve your problems without taking the time to test east individual fixture and cable in the line. This looks like a connector with no cable attached and has a 120 ohm resistor between pins 2 and 3. This prevents any signal bouncing back from the end of the line–signal noise that can cause lights to act unexpectedly as described.

If there are many lights in the line (let's say more than 15 or so), some of the signal can get garbled, especially if using xlr cables. The best solution for this is to use a DMX splitter so that you can split a line of 40 fixtures, for example, into four lines of 10 fixtures.

[jimmol]

DMX terminator first and see if it solves your problem, then divide and conquer, as Peter said. Hopefully that will solve everything.

The person that made your DMX cables should be able to wire you up an DMX terminator or check Amazon.com and order one.

I believe Chauvet makes a Data Stream™ 4 splitter which I use and the others around here may have used other splitters that they might recommend.

Best Wishes

[unc417]

Most of the lights and cabling were installed 3+ years ago. A few months ago we added more fixtures, nearly 30 now in the chain. Though I've never had a terminator and have never had an issue until now. I reseated all the connections and did not have any issues in the 1 performance I've had since doing that. I do plan on getting a splitter to reduce the number of fixtures in the chain but don't have the money for it yet.

[daver2]

You may find this thread useful unc417.

Can XLR work instead of DMX?

I always cite cable and terminators first as these (or lack of them) are usually the culprit!

The DMX protocol requires a terminating resistor of about 120 ohms to prevent reflections on the line from corrupting the data communication. As Peter Coleman has stated (by the way Peter - welcome to CMN as this is your first post) this is only a 120 ohm resistor (cost a few pence/cents) wired to a standard connector. If you look at the end of the thread I have given you - you will find references to websites which show you how to make your own terminator and (also) how to make a useful DMX monitoring plug from a resistor and an LED.

The trouble with not running your equipment with a terminator (or the wrong type of cable) is that it will work fine most of the time. Then, as you add more equipment or something else changes (e.g. the photocopier is changed or moved), the increased electrical interference will start to affect the DMX communications and things will start to behave erratically.

Dave