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Finding Fault with DMX - TvTechnology

Finding Fault with DMX

Despite having been a part of the lighting scene for almost 20 years, the DMX512 control protocol still seems to be shrouded in mystery for far too many of its users.
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Despite having been a part of the lighting scene for almost 20 years, the DMX512 control protocol still seems to be shrouded in mystery for far too many of its users. The key facts are that DMX512 consists of packets of lighting information sent at 250 kbps, over an RS-485 (now TIA-485) data link. Each packet of data contains a single identifying byte to indicate what the packet contains, followed by a block of information for use by receiving devices such as dimmers, moving lights, effects machines, media servers, etc.

Long before lighting control went digital in the mid-'80s, the TIA-485 serial data protocol was in regular daily use for reliable data communications and industrial control applications. That's what made it such a good choice as the medium for carrying DMX512.

As a transmission line system, the basic electrical parameters of the TIA-485 line are quite simple: The line should be shielded twisted-pair cable in a continuous bus with an impedance of 120 ohms and with 120-ohm terminations at each end. Up to 32 receiving devices can be connected to the bus with a maximum distance of 1,600 feet between devices, and maximum overall bus length of around 4,000 feet.

On top of this, the DMX512 standard specifies that connectors XLR style 5-pin, with pin 1 as the common reference, pin 2 as data-minus and pin 3 as data-plus.

Most of the problems with achieving reliable DMX512 data transmission in a production control system arise from the failure to follow that simple recipe.

Those of us who work in television are already familiar with the discipline imposed by the 75-ohm coaxial transmission line system that is composite video. We have seen the results of attempting to T- or Y-split a video feed without the assistance of a distribution amplifier. We are also familiar with the consequences of not terminating the end of the transmission line, or terminating it more than once. And most of us would never dream of using a set of adapters to transmit composite video over a zip cord or a power lead.

UNSUITABLE CABLES

(click thumbnail)The Micro-Scope DMX tester from Artistic Licence
Regardless of this knowledge, a significant majority of problematic DMX networks use a resoundingly unsuitable type of cable--microphone leads. While the signal from a DMX512 transmitter, such as a lighting controller, is a square wave at 250 kHz, audio cables are only required to perform in the range of 20 Hz to 20 kHz, and thus tend to attenuate and skew the DMX signals that are nearly four octaves above the cable's intended operating range.

As mic cables are also of the wrong characteristic impedance, there are signal reflections at the points where they join to cables of different impedances, such TIA-485 cable or even other brands of mic cable.

One primary source of confusion over the suitability of mic cables for DMX512 is that several major moving light manufacturers have XLR-style 3-pin connectors for the control input on their fixtures. While long ago, fixtures needed to have plugs suitable for the manufacturer's proprietary controllers (which did connect via standard audio cable), virtually none of the current generations of moving lights can actually run off such controllers, and therefore have no excuse for being fitted with the 3-pin connectors.

Although a few production and rental companies use 3-pin to 5-pin adaptors on their 3-pin DMX equipment, most simply have stocks of 3-pin DMX cables. From there, it's only a short journey to inadvertently picking up a mic cable here and there and including them in the rig. The results are often unpredictable and usually sufficiently intermittent as to appear to be totally random.

Of course, once you are using mic cables, it's easier to borrow a couple of Y-cords from an audio equipment box to split the DMX512 signal than to use an expensive electronic DMX splitter that also requires a power feed. Unfortunately, as soon as a TIA-485 transmission line acquires branches, there are multiple points for data reflections, and digital chaos ensues.

TIA-485 receivers, such as those in dimmers and moving lights, look for voltage changes in the line to interpret as the ones and zeroes of digital data. They are unable to differentiate between the signal sent by a controller such as a lighting console, and the reflection of that same data bouncing off either a Y-splitter, a cable impedance change where a mic cable has been inserted, or the unterminated end of the transmission line. Any one of these physical network errors can produce similar unpredictable network behavior.

CALL OUT THE CAVALRY

How then can we identify the source of data errors in an apparently randomly misbehaving network? The first step is to make certain that there is a termination at the far end of each separate transmission line in the DMX512 network. (In DMX systems, the near end is terminated internally by a controller or a DMX data splitter.)

The next move is to physically confirm that all branching in the network is a result of functioning data splitters. If this yields no enlightenment, then each branch on the network needs to be checked for the possibility of rogue audio cables.

At this point, if you still haven't resolved the source of your network problems, it's time to call out the cavalry--a DMX512 tester (not to be confused with the DMX detectors that consist of an XLR plug fitted with a couple of LEDs). These devices can act as either a generator of DMX signals to inject into the network, or a receiver of DMX512 to analyze the data stream at a particular point in the network. There are a multitude of such testers on the market from such companies as Lite-Puter, American DJ, Martin, Artistic Licence, Goddard Design, Zero 88, Anytronics, ShowTech, GAM, Soundlight, MicroTech, Dove Systems, Mad and Swisson, to name but a few.

Probably the most comprehensive, versatile and, at $850, most expensive of these devices is the Lil'DMXter from Goddard Design in Brooklyn, N.Y. Considered by many as the DMX reference device, I have been using the Lil'DMXter for about 15 years, and aside from its rather corny "cute" menus, it has proven to be an invaluable aid in resolving DMX512 network faults.

Having the right test gear for the job is essential, but having the right method for locating where to test and what to test for is absolutely vital. That method is the topic for my next column. In the meantime, if you do have an erratic DMX network, it's well worth looking carefully for cabling that doesn't belong. It's time to give the audio department back all of those cables you borrowed.