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PROTOCOL CONTROL AND ANCILLARY ENABLERS

PROTOCOL

DMX

The Digital Multiplexing (DMX) interface is a low-voltage signal that distributes data between the control console and dimming. Its use in lighting was initially a simple way to use any console with any dimmer, a standard that originated in 1986 by the USITT. However, DMX uses grew beyond anyone's initial intentions, and it became the primary method not only for linking a console to a dimmer but also to link multiple signals sent and received from an offstage or truss location to entire lighting rigs of moving luminaires, hazers, strobes, light-emitting diode (LED) fixtures, and digital lighting.

Each DMX universe is 512 channels and is wired using a 3-pin or 5-pin connector. DMX is the official universal protocol for networking concert lighting as developed by the United States Institute for Theatre Technology (USITT) and the Entertainment Services and Technology Association (ESTA) and codified by the American National Standards Institute (ANSI).

Because DMX can be linked from one fixture to another in a “daisy chain” configuration, it reduces the amount of cable running to the distribution location. Each light does not have to have a separate line, but the link cannot exceed 512 channels, either. Some moving luminaires can have 20, 30, or more attributes that must be controlled, and once the maximum of 512 channels is reached another universe is required to control anything further. There is no limit to the number of universes that can be used, assuming that the capability is available via the lighting console or other device.

With a large lighting rig, or even a small one, comprised of widespread fixtures that require DMX control, it is not always possible or practical to daisy chain the entire system. The daisy chain can give rise to several troublesome issues when linking multiple DMX runs together. A lot of time can be wasted trouble shooting problems that often occur, even for a simple point A to point B DMX cable run. One bad cable in a daisy chain can disable an entire row of fixtures. Ways to converge DMX from multiple locations are discussed below in the Ancillary Enablers section.

ACN

The Architecture for Control Networks (ACN) is the new kid on the block that may replace DMX as the control protocol for lighting systems. It was developed by ESTA and its development body, Control Protocols Working Group. ACN is a suite of network protocols for high-speed, bidirectional communication over standard Transmission Control Protocol/ Internet Protocol (TCP/IP) on an Ethernet network infrastructure.

A primary goal of ACN is to provide a reliable transport mechanism, which it does through Session Data Transport (SDT), which transports data to ACN lighting components and devices. Once a session has been initiated in SDT, data are efficiently packaged n varying types and sizes and transferred back and forth. ACN also includes Device Description Language (DDL), which is a text language that describes features of a device—for example, a moving luminaire with several attributes. DDL allows devices to tell controllers how they would like to be describedand controlled, which eliminates the need for luminaire libraries that currently are separate attributes, such as pan, tilt, and focus.

ACN is most usable for controlling more complex devices such as media servers and audio mixers. It has also been proposed as the primary transport for HD-MIDI, a protocol that enables electronic musical instruments, computers, and other equipment to communicate with, control, and synchronize with each other. The gist is that ACN is an open-ended suite of protocols that is cleverly packaged and effectively used by network devices to transfer data with greater and more adaptive control in theatre applications. ACN provides a fast and efficient mechanism to transport the well-understood DMX protocol over Ethernet in an open, industry-standard way to control a variety of entertainment systems.

RDM

Remote Device Management (RDM) is a protocol enhancement to DMX-512. RDM works along with DMX-512 to provide two-way data communication allowing for remote device management. It uses the same wire pair as DMX-512 but requires bidirectional devices in the data path for the RDM traffic to get through. In other words, the luminaires or other devices must also allow for such two-way data to be transmitted. Most of the newer lines of luminaires and consoles are adding this capability. Basically, this allows you to use remote-control devices in the DMX line with all of your other controllers without interrupting them. This protocol allows configuration, status monitoring, and management of these devices. It has a small impact on the DMX refresh rate, but should not disturb the normal operation of standard DMX devices that do not recognize the RDM protocol. RDM was approved by ANSI in 2006 and is rapidly gaining popularity.

ANCILLARY ENABLERS

The idea is that we want a trouble-free, streamlined system that uses as little cable as possible. There are around 14 manufacturers that develop and produce reliable and convenient electrical boxes designed to split, isolate, adapt, convert, interface, and distribute

DMX-512 transmission signals. We refer to these as ancillary enablers. More computer protocols integrating into lighting control mean more electrical boxes to contain, manage, and organize the amount of DMX cable running to distribution locations. These ancillary enablers are essential tools that improve control, data efficiency, and cable management in concert lighting.

Optosplitter

Because DMX-512 is an electrical (electronic data) signal, it must be isolated from other data lines and protected from any power surges than can damage expensive lighting and other DMX controlled devices. The Opto splitter is a small electrical box (Figures 20.1 and 20.2) with multiple outputs that provide data line separation for DMX receivers such as dimmers, color changers, and moving lights. This is a star configuration as opposed to a daisy chain configuration. In a star configuration, each controlcable is run to a central point—in this case, the splitter. In a daisy chain configuration all the devices are connected on one control cable, the output of one feeding the input of the next.

Terminator

A terminator is a resistor that matches the impedance of the cabling used to join pins 2 and 3 of the connector. It is a small, pocket-size, 3-pin or 5-pin connector that plugs into the DMX-512 out connector at the end of a DMX chain. Without a terminator, signals can reflect back down a cable; the initial signal combined with the reflected signal causes it to see two numbers at once, resulting in a faulty signal.

Adapter

An adapter is an enabler. It is a short cable (usually about 6 inches) with a 3-pin connector on one end and a 5-pin connector on the other (or a 5-pin to 3-pin double connector part), and it is used for switching cables at the distribution end or the receiving end. Adapters can be a timesaver if you have accidentally run the DMX cable the wrong way, or they might be necessary when using 5-pin cable but the Opto splitter has a row of 3-pin outlets.

The DMX-512 standard provides for 2 data links in one cable, which only uses 3 of the 5 pins for data, making a 3-pin adapter possible for a 5-pin DMX cable. Line 1 is on pins 2 and 3, line 2 is on pins 4 and 5, and pin 1 is a shield for the cable. A cautionary reminder: DMX standards require the use of 5-pin connectors for lighting cables so they will not be confused with the 3-pin cables that sound systems use. Sound cable has different impedance and capacitance specifications for data bandwidths required by DMX-512. Among a few adapter options not listed here, every lighting tool box should have a selection of adapters or spare connectors that can easily be used onsite to accommodate any adapter situation.

INTERFACES

An interface is an enabling electrical box for those times when a device is not DMX-512 compatible.

There still are a few old proprietary protocols being used in the lighting market. If the owners haven't upgraded those devices, it would be impossible to use a DMX console with a proprietary dimmer protocol, for example, without an interface that was specifically designed to make the necessary conversion.

Wireless DMX

Considering that most front-of-house lighting console positions are between 100 and 300 feet from the backstage dimmers and power distribution centers, the advent of wireless DMX is certainly a welcome development, saving time and carrying long runs of cables. It works by having the DMX-512 control data from any standard DMX-512 console output attached to the wireless distribution system (WDS) transmitter. The transmitter broadcasts the signal to the WDS receiver (or receivers). The WDS receiver then outputs the signal as standard DMX-512 data, and the unit can be connected with standard cables to WDS dimmers or other devices, such as moving luminaires or effects.

With so many wireless computers, cell phones, and wireless microphone frequencies competing for air space in a venue, indoors or outdoors, what if the frequency that controlled an entire lighting system dropped out or was interrupted in the middle of a show because the frequency was overcrowded? City Theatrical, Inc. (CTI), thought of that eventuality and in October 2008 was granted a U.S. patent for its wireless technology (Figure 20.3). CTI's patent covers the transmission of DMX, RDM, and ACN via frequency-hopping spread spectrum (FHSS) radios, as well as direct sequence spread spectrum (DHSS)

radios, in the 2.4-GHz range. Also covered are bidirectional broadcasts of data and the concept of a remote monitor for RDM and ACN. CTI president Gary Fails explains that, “Most other wireless DMX radios hop randomly, relative to the DMX packet, breaking each DMX packet into pieces and stitching them back together in the receiver. Individual data bits and even entire packets are frequently dropped and lost, causing older data to be substituted for

the lost data. In some situations, such as incandescent loads or static levels, this isn't a problem, but in demanding cases, such as moving luminaires or LEDs, data loss can be quite obvious.”

Ethernetto DMX

When I started in the lighting field, words like computer, software, and laptop were not even in the vocabulary. Now, Ethernet, yet another computer-based technology, has joined the lighting world to streamline data and cabling. Ethernet cables carry far more information than DMX. Electrical boxes that are designed to convert Ethernet into DMX are enablers. Unlike DMX transmission signals, Ethernet is frame-based computer networking technology. A frame is a digital data transmission unit that allows data to be transmitted in larger packages instead of signals. In this way, a device that uses CAT5 Ethernet cable (which carries information to the Ethernet port) on one side is able to convert those signals into DMX outputs that vary from one to six or more, depending on the manufacturer.

Simply put: Ethernet cable into a converter box, results in traditional DMX out (Figure 20.4). DMX-512 can also be converted into an Ethernet data format for distribution and routing, but it needs to be reconverted into a standard DMX-512 form for connection to DMX-512 devices.

See Figure 20.5 for protocol, control, and ancillary enabler tool manufacturers.