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Troubleshooting DMX 512

Richard Cadena • Light On The Subject • November 1, 2009

If terms like “parity errors” or “overflow errors” mean nothing to you, seek help from books like these, or other DMX guides.

For best results, leave the audio gear in the toolbox

Once upon a time, in a land not so far away, there was a lighting consultant who received a call from a theatre who wanted help. The theatre had just purchased its first automated lights and they asked the consultant to help them program the console. The lighting consultant was happy to comply, but he made several requests of the theatre in order to make the best use of the time. The first was that all the fixtures should be tested and the DMX512 addresses set before they were rigged and flown. The second was that the fixtures should be rigged, powered and connected to the DMX512 network before the programming started. The theatre thought the requests were reasonable and they complied.

When the consultant arrived he saw that all of the fixtures were hanging in the air and he proceeded to test them. First he checked the status indicators on the fixtures. The power indicators were on solid but the data indicators were not. Since the console was not on yet, that was normal. He also looked at the DMX512 addresses and saw that they were all set correctly.

Next, he went to the console, fired it up, and went into the setup menu to patch the new fixtures. He found the fixture profiles, loaded them into the patch, and configured the DMX512 addresses correctly. Then he got out of the set up menu, selected all of the new fixtures, and homed them. But instead of all of the fixtures going to the home position, some went off in odd directions, some started wigging out and some did nothing. Based on what he saw, the consultant said that there must be a bad data cable.

The theatre techs were in disbelief. “We built and tested all of the data cables ourselves,” they said. “What could possibly have gone wrong?”

After a bit of detective work, the consultant knew exactly what went wrong.

Building and Blessing in Cableland
When the techs built the data cables, they had a young intern solder all of the terminations. After the cables were built, they tested and blessed each one of them before they installed them. But because they were audio techs too, they used an audio cable tester, not a data cable tester.

An audio cable, which is designed to carry analog signals of relatively low frequency, is very different than a data cable, which is designed to carry high frequency digital signals. By the same token, an audio cable tester is very different than a data cable tester. An audio cable tester typically uses a DC source, like a 9V battery, and checks for continuity. But a DC voltage might act very differently than a high frequency square wave, which is what a digital data signal is. Only a data cable tester can tell you with any amount of certainty whether or not a cable is good for carrying digital data. Why?

First of all, if the cable exhibits characteristics of a capacitor then it will render a digital signal indecipherable. And a cold solder joint can act like a capacitor to a high frequency data signal. So can a microphone cable because it’s constructed differently than a data cable.

Second, a cold solder joint represents a change of impedance, which can cause data signal reflections. For an audio signal that may or may not result in audible distortion but to a data signal it can cause signal cancellation corrupt the data.

For reliable DMX cable testing, you’ll need the right tool. Use a data cable tester, like the Swisson XMT-120, pictured here.

Real Data Testers for Real Data
A continuity checker may or may not find problems that a digital data signal will. There are DMX512 testing devices made by several manufacturers including Artistic Licence, Doug Fleenor Design, Goddard Design, and Swisson, each of which has a cable test mode. Artistic Licence’s Micro-Scope, for example, is a battery or wall-powered handheld device that performs a variety of DMX tricks including testing cables. In cable test mode there are three different methods of operation. In the double-ended cable test, both ends of a data cable are connected to the tester and it verifies that there is AC continuity between corresponding pins on either end of the cable and that there are no shorts. In the single-ended test mode only one end of the data cable is connected to the test and it verifies that there are no short circuits between pins 2 and 3, which carry the data signal and its complement.

The network test mode is the most sophisticated of the three test modes. It uses a special function of DMX512-A, which is the latest version of the standard, to send a test packet of data for the purpose of analyzing the integrity of the network. It is sent by a transmitter and analyzed by the receiver to make sure the information was received correctly. With the Micro-Scope, the tester is connected on the console end of the cable and another on the receiving end receives the DMX512-A test packet, analyzes it, and indicates whether or not the data is good or bad. This not only insures that every component in the link is working but that it is maintaining the integrity of the data.

Goddard’s MiniDMXter and Doug Fleenor Devices’ Gizmo are two more examples of battery-operated, handheld DMX testers. Both check for continuity on pin 1 between both ends of the cable (indicating that the cable is plugged in) and then sends short bursts of DMX512-like data. If the data is received properly it confirms the proper pin connections at the other end of the cable. If the cable has pins 2 and 3 reversed or if there is high capacitance it will tell you so.

Swisson’s XMT-120 is yet another option for testing DMX data cable. Like the other testers, the XMT-120 can transmit and receive DMX512 data for a variety of reasons. If you want to check an operating DMX512 network you can start at the end of the data link and plug in the tester. It will display any channel of the incoming data in percentage from 0 to 100%, decimal values from 0 to 255, or in hexadecimal from x0 to xFF. If you start at the end of the run and find that the data is corrupt or missing, then you can start working back to the console and insert the tester between each successive data cable and the previous fixture until you find the problem cable.

My Friend Flicker Finder
Some of these testers, including the family of DMXters (MiniDMXter, Lil’DMXter, and the DMXter4 RDM), the Micro-Scope, and the Gizmo have a “flicker finder,” which allows you to find intermittent problems in a data network. In flicker finder mode, the tester is plugged in somewhere in the data link and receives DMX512 data. The console has to be set to output the same channel values and it can’t change. If there is a change in the value of the data the tester will indicate that an error has occurred. This is helpful for testing the integrity of the data over time. But the console has to be in one cue and cannot change any DMX512 values in order for this to work. Therefore, it cannot be used during a show. If it’s important to monitor the integrity of a DMX512 network during a show you can insert a tester somewhere near the start of the DMX512 network and it will interleave test packets along with the DMX512 data. By plugging in another tester at the end of the data link you can monitor the network live during a show.

These DMX512 testers vary in price from $356 for the Swisson XMT-120 to $1,296 for the Doug Fleenor Devices Gizmo but they are invaluable for troubleshooting DMX512 networks. Many of them have so many more features and functions that the cable tester is just scratching the surface. For more advanced troubleshooting some of these devices will find parity errors, framing errors, break and mark after break timing errors and overflow errors.

If these terms are foreign to you then you should immediately seek help. It can be found in such books as Control Systems for Live Entertainment by John Hungtington, Practical DMX by Nick Mobsby or Recommended Practice for DMX512, 2nd Edition, by Adam Bennette.

You wouldn’t use an audio amplifier for DMX data distribution so why would you use an audio cable checker to test a data cable?

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