Testing dmx cable

[RickR]

FYI:
DMX standard calls for 32 devices maximum. If you have more then a splitter is required. If not then a splitter only isolates or masks the problem.

DMX is based on EIA/RS-485 that requires about 120 ohms impedance, but the exact equipment will determine how close to that you actually need to be for it to work.

Terminators help maintain the impedance within the accepted range. In some cases they are either not needed or a higher (double) value is needed. Thus networking gets really complicated. However as Daver2 says, you can buy 120ohm 1/4W resistors at Radio Shack for a buck or so and get quite a ways by trial and error. It goes between pins 2 & 3.

Fixtures have a DMX "transceiver" chip. If it gets fried (by audio signal, static, microplex etc.) then it can garble your whole system.

[james tucker]

DMX splitters are a blessing.

I've walked into plenty of places with 24+ fixtures on the same DMX line.

When I am doing shows I take my Elation splitter with my console to split the universe into 4 "regions" :
1 - upstage truss
2 - downstage truss
3 - on stage fixtures
4 - hazer

You can have the happiest DMX cable and termination in the world... if a random LED, mover, etc has a bad solder point from the xlr connector to the logic board, it will cause havoc.... By splitting things, its easier to spot and isolate problems. Your ebay led Par lighting up the drums on the floor won't screw up your movers in the house.

I bought one of THESE a while back... It tests 5 pin and 3 pin connectors. Its nice to keep in your toolbox.

[Esoteric]

I think I may blog on basic DMX troubleshooting today.

[D.R.HADDOCK]

Some dmx controlled lights with master / slave configuration options
will use non standard dmx protocols for there master slave communicatons,
these non standard protocols can sometimes upset the dmx system
causing random flashing.
These lights will use non standard protocols usually for a short period
during inital start up.
How ever if you have a doggy connection somewhere in your dmx line
and there are temporary drop outs,
then these lights could be attempting a start up reset
because they have lost there dmx input.
Start by jiggling all your dmx leads, and see if any problems arise.
There could well be a faulty lead/plug somewhere.