Length of DMX cable a factor?

[lanes6]

I recently hooked up a 4 channel dimmer pack and 11 LED par 64's in our sanctuary. After I finished I decided to add 5 LED 38's as uplighting on our stage. I am not getting a control signal to those lights. I'm suspecting that my cable has run too long (I am about 330 feet) because when I hook up a short test cable from my last 64 to my 38 it is controllable. Is there a maximum length of cord for signal to reach? If so, what can I do to resolve this issue? I am running computer controlled DMX, American DJ interface, with USB adapter.

[EmilBarnabas]

Do you have a terminator at the end of the DMX line? Make your own by soldering a 120 Ohm resistor between pins 2 and 3 inside a male XLR connector. Stick that in the last fixture.

You do have everything in one long daisy chain? No Y cables.

You are using good quality DMX cable, not microphone cable?

Your DMX chain should be able to extend up to 3,900 feet. (using the right cable and terminator)

The official spec says that you can have only 32 devices on a DMX chain.

[waynehoskins]

Always use good data cable, especially for long runs. Pretty sure that's what the problem is; if you're using microphone cable, by the end of the run there's likely either too much voltage drop or waveform rounding for the digital control signal to be recovered.

[lanes6]

I was told that all of my cables were for data. I bought a roll of wire with the lights and made some of my own cables too. All lights are daisy chained, no Y's. Never heard of putting a resistor in the last light, what does that do? I don't have a resistor at the end of my current set up and it seems to be working well.

Is there some type of signal booster for problems like this? When I plug in the last five 38's it causes some of my 64's to flicker. I have a 60 foot run from my last 64 to my first 38.

I'm no where near 3900 feet and way under 32 devices. Sounds like a cable problem, huh?

[bladeaudio]

Try the terminator at the end of the line. The DMX specification calls for the line to be properly terminated.

[petereit]

Definitely terminate the last fixture on your chain. If you still have a problem, start at the end of your chain and disconnect one fixture at a time (making sure you move that terminator) until your fixtures are all operating normally. At that point, your culprit is either the last fixture you disconnected or the DMX cable that ran to it.

[Esoteric]

Yeah, you are no where near the max length for a DMX run. You can try termination (many cheap LED PARs don't self terminate). You might have a bad link in the chain as well.

Mike

[daver2]

You can use any old cable and not have a terminating resistor at the end of the DMX chain - and things will probably work OK for small installs. Extend the installation (or increase the ambient electrical noise) and you will start to get 'glitches' and the system will start to become unreliable. Increase the install further - and things will stop working or become so unreliable they are unusable.

The Standards are in place to ensure that an install should work properly if they are adopted by the installer. You state that you have bought the correct cable. To confirm that it is data cable (and not microphone cable) around the sheath of the cable should be some identifying marks. Google these and identify that the cable is, in fact, data cable.

The DMX512 Standard mandates that the last fixture contains a terminating resistor. Some fixtures (more upmarket ones) automatically terminate the line themselves if nothing is plugged in. The cheaper fixtures expect you to add a terminator yourself. A previous post has already explained the cheapest way to make a terminator with a 120 ohm resistor.

The terminating resistor is to prevent reflections from the end of the line. Drop a stone into a pool of water and watch the waves when they hit a solid object (like the edge of the bath) - they reflect off the solid object and interfere with the original wave. The resistor is the electrical equivalent of preventing this from happening.

Ensure the install is up-to-specification first and then take it from there.

Some useful reference websites:

http://www.dmx512.com/web/light/dmx5.../term/term.htm

http://dmxit.com/dmx_512_standard.htm

http://en.wikipedia.org/wiki/DMX512

Dave

[lanes6]

Stupid question: which leads are 2 and 3? I'm using 3 post ends.

[waynehoskins]

They should have numbers molded in the plastic by the pins or sockets, but if they don't, here's a trick:

Viewed holding the latch up:
Pin 3 is the single bottom pin.
Pin 2 is the pin on the top row beside the keying notch/ridge in the connector shell.
Pin 1 is opposite the key.

[EmilBarnabas]

Wikipedia has diagrams showing the pin numbers of both the female and male XLR connectors.
http://en.wikipedia.org/wiki/XLR_connector

[lanes6]

I figured out what my problem was: When making my own DMX cable I didn't adjust the wire so that the #1 and #2 post continued with the same wire into the other end. I got them switched around. Newby dumb. Thanks for all your suggestions. Everything is working perfectly! Don't know why it was even working at all with messed up wires.