Wiring as follows:

The Bi-Color led has 2 LED's (one Red and one Green) back to back in the same package. Build this into the back of a 5 pin XLR with the LED showing out of the back. To use insert into the socket that you want to test, Set all channels to 0% (Preheat also = 0) The led should glow one color. Run all channels up to Full power (You may need to turn the dimmers off!) The LED should glow the other color. If the led fails to light in any one of the 2 above states then there is a short between one of the lines and pin 1.

Connectors

The DMX standard specifies 5 Pin XLR style connectors, However only 3 pins have standardised uses. The remaining pair (Second data link) are used for anything from Overtemp. indication to supplying power to control desks! If using equipment from more than one manufacturer check the spec. carefully as to the use that is made of these pins, it is generally best not to connect pins 4 and 5 if you are not using them.. Several manufacturers use 3 Pin XLR's for 'DMX' This is not standard. It may be worth having several jumpers to convert between these (and to isolate the second data link).

Splitting a DMX feed

You cannot split a DMX feed by simply soldering 3 cables together. Doing this may work some of the time but it cannot be relied upon. Most DMX compatible equipment has a DMX out socket fitted and this may be used to loop into other equipment. If you need to split a DMX feed a splitter box can be used (see the Compulite pages) These can also provide isolation such that a fault on one line does not cause a malfunction on the other line. An opto isolator is highly recommended on lines feeding dimmers as a fault could destroy other equipment on the line. Note that a line from an isolated port is considered to be a new DMX line.

The EIA 485 standard supports a maximum of 32 devices on a line. If more are needed a repeater is needed. This will allow another 32 devices to be added for a total of 63 devices (The repeater counts as a device). This can be continued for more devices. However where large networks are required it is better to use a splitter to produce multiple lines at the source. This means that if one line fails the problem will be limited to that section.

It is also worth noting that not all instruments have "proper" RS-485 receivers in them. Some of the wiggly lights (early I-beams?) simply stuck the LED in an opto-isolator across the DMX line, rather than a real receiver. This is OK, as long as you are only driving ONE thing from the line, and the line isn't too long. I think these things daisy-chained, with each light regenerating the signal for the next instrument.

Other protocols

These are details of a few other protocol in use, none of these are compatible with DMX.
* AMX 192 (USITT) : This analog TDM protocol was used before DMX became common. It suffers from all the problems inherent in any analog protocol (Earth loops, Noise). It is carried on 5 Pin XLR.
* D54 (Rank Strand) : This is also an analog TDM protocol. It is single ended and VERY prone to earth loop problems. If you have problems with this one they can usually be solved by making sure that the desk, Monitor and Demux are at the same ground potential. You sometimes get REALLY weird behavior with this, Compulite produce a converter to DMX which can correct some of the problems, see the Compulite pages.
* PMX (Pulsar) : This protocol is EIA 232. We do not have any more details.
* CMX (Colortran) : DMX was based on this (Amongst others). CMX equipment can usually be easily modified for DMX.
* C105 (Compulite) : An early digital protocol, had the advantage of an opto-isolator board in the output stage of the control desk which was powered from the demultiplexer, thus reducing noise problems.
* CMX (Compulite) : This is based on DMX, but uses the second data pair for return of status information from their digital dimmers. Original versions were NOT compatible with DMX, but all systems from mid-1998 on produced a CMX output which was fully DMX compliant.
* SMX (Rank Strand) : Designed to provide fault reporting and other nice features, unfortunately it is so complex that it is seldom seen in practice.
* Midi Show Control (MSC) : Not really a lighting control protocol, but so common on large shows that I have decided to put it in. It provides show control functions, like synchronising sound and lighting with pyro. This is used BIG TIME in the theme park industry.