DMX TERMINATOR?

[soundmanjim]

Ok, i have 3 showtec multidims controlling 3 groups of par56 cans; the last dimmer in the chain has a sound to light function which the others dont have but will make them all flash in sympathy. (sort of)
Not that im bothered about it as i control the lights from the mixing booth, but do i need a dmx terminator on the last dimmer in the chain?

[soundmanjim]

anyone?

[soundmanjim]

DMX512 cable is used to link a DMX512 controller to many DMX512 fixtures. Each fixture has a DMX512 in and usually a DMX512 thru connector. The DMX512 in on the first fixture is connected to the DMX512 out connector on the controller, then the DMX512 thru is connected to the DMX in of the second fixture, and so on. The final, empty, DMX512 thru connector should have a DMX512 terminating plug plugged into it. A DMX terminator is a 120 ohm resistor between pins 2 and 3 of the connector. Fixtures therefore 'daisy-chain' from one to the other.
( i eventually found it out myself!!)

[pete]

;D yes you need a DMX TERMINATOR for the last dmx dimmer in line or other electrical ie mobile phones etc etc can interfere with the lights (i was told by a lighting salesman in a shop therefore i always used em as i am new to the game.

I have 2 sets of 4 par 56 , i also run on a third dimmer 2x strobes 1 x waterworks fx 1 x flower gobo fx and then i got two mojo spin masters on a dmx connection as well as smoke machine.. i have not had any problems with them whatsoever

i am lucky i guess ;D

i run me lights through the Behringer LC2412 DMX Control Desk

[John Bell]

Hi Pete,
those Behringer boys have a piece of kit for everything these days. I am looking at replacing my 3x Ryger DX403 controllers with a 24 channel lighting desk and dimmer packs, for my 20x par56 and 2x NJD Quartet colour changer flood-lights. What dimmers are you using?

[Simon Ryder]

Actually, the 120 ohm resistor increases the resistance of the over all circuit which is necessary to prevent the fragile DMX binary code from "bouncing back". Basically if your system has the wrong resistance then the DMX signal can interfere with itself as it reaches the end of the line and returns. This is nothing to do with picking up interference or magnetic induction (otherwise DMX leads would be shielded) but is due to the need to stop the signal "returning". This is why you find that digital audio signals use 75 ohm cables for SPDIF and 110 ohm for AES/EBU and the lampies use 120 ohm teminating resistors for DMX leads. As DMX uses either a 5 pin or a 3 pin XLR (only 3 of the 5 pins are used) you will quite often find mic leads being used instead of DMX cables. This works for the lampies because, even though the actual DX leads have a slightly higher impedance than mic leads, they use the terminating resistor to set the overall resistance. Needless to say though that we can't use DMX leads as mic leads due to the slightly higher impedance giving a lower signal and the fact that they are unshielded (just having an earth wire) and so will hum horribly.

Hi John,
For dimmers either buy second hand - much cheaper though you may need to add a analogue to DMX converter or try Showtec who are distibuted by Highlite International - www.highlite.nl and their UK office phone no. is 0124 981 9494.
Regards,
Simon

[Salvador]

just stick a resistor (dont remember how many ohms in the last DMX out of the chain (sry can't remember the pins either)

Salvador

[soundmanjim]

its all done, although i still havent worked out how to program a flippin' chase into the bellinger yet!!!