DMX 512 FAQ

DMX 512 is RS485 serial at 250 Kbaud, as such any installation must meet the requirements of EIA-485. In particular use of suitable good quality cable is important (Mic cable is NOT suitable) suitable cables include :

• Belden 9841
• Belden 9842
• Alpha 5274

Amongst others.
This is not a complete list.

RS485 is NOT the same thing as RS232 It cannot be made from a standard PC serial port. You can damage a DMX device by attempting to plug RS232 into it!

3.1 Wiring practice

At the speed DMX works correct wiring practice is important. Always arange wiring such that the data source is at one end of the cable. NEVER do this :

           *----*-----*------------*---------*
                |                  |
                *                  *
                                   |
                                   *---*---*
                                   |
                                   |
                                   *---*

Where * is a DMX plug-socket pair.
I have seen this done! It worked (sort of, sometimes), but would fail in unpredictable ways when extra devices were plugged in. DMX cannot be split by simply soldering 3 cables together. Daisychain everything, but see the section on isolation.

3.2 Line termination

All DMX lines must be correctly terminated if reliable operation is to be obtained. Some equipment has a switchable line terminating resistor built in. In these cases make sure that only the last item in the chain has its terminating switch set to on. If the last item (Farthest from data source) does not have a terminating switch then a external line terminating network is needed. To make one you will need the following parts :

1 Male 5 Pin XLR.
1 110 ohm 0.5W resistor.

To make the line terminator solder the resistor across pins 2 and 3 of the XLR. You may like to back fill the XLR with potting compound or epoxy resin to provide protection.

3.3 A simple DMX tester.

This tester can be used instead of a line terminator to provide a simple means of checking for cable continuity.

Parts required :

1 Male 5 Pin XLR
2 270 ohm 1/2 watt resistor.
1 Bi-Color LED.

Wiring as follows :

  2>-----------+------------+
               |            |
               -            -
              | |          | |
              | |          | |
              | |          | |
               -            - 
               |            +
               |            |
               |           LED
               |            |
               |            |
               |            |
  3>-----------+------------+

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 chanels 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.

Its limited but suprisingly useful!

3.4 Connectors

The DMX standard specifies 5 Pin XLR style connectors, Howether 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 then one manufacturer Check the spec. carefully as to the use that is made of these pins. 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).

3.5 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 compatable 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. These can also provide isolation such that a fault on one line does not cause a malfunction on the other line. A opto isolator is highly recomended on lines feeding dimmers as a fault could destroy other equipment on the line. Note that a line from a isolated port is considered to be a new DMX line.

These can be obtained from any of the usaual suppliers of DMX hardware, or they can be made cheaply by anyone with a electronics background. 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. Howether 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 wiggle 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.

3.6 Other protocols

Some short notes on other common protocols.

• AMX 192 (USITT) : This analog TDM protocol was used before DMX became common. Mostly found in the US. 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 a analog TDM protocol. It is single ended and VERY prone to earth loop problems. If you have problems with this one they can usaually 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.

• PMX (Pulsar) : This protocol is EIA 232. I do not have any more details.
• CMX (Colortran) : DMX was based on this (Amongst others). CMX equipment can usaually be easily modified for DMX.
• SMX (Rank Strand) : Designed to provide fault reporting and other nice features, unfortunatly 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 syncronising sound and lighting with pyro. This is used BIG TIME in the theme park industry.