Prehistoric man used primitive lamps made from naturally occurring materials, such as shells, horns, and stones, to illuminate his cave. Commonly filled with grease, these lamps had a fiber wick that was fueled with animal or vegetable fat.
Early man also realized that carving a niche into the cave wall would help to direct and intensify the light, discovering the first crude lighting control by reflector principle. Hundreds of these lamps (hollow worked stones) and niches have been found in the famous caves in Lascaux, France, dating back to about 13,000 BC.
While the control of light intensity and beam direction still remain the two most important properties of modern lighting instruments (which is the correct technical term for light fixtures or luminaries), more functions such as projecting and changing colors, the projection of patterns and symbols, and the variation in focus, as well as strobe effects, have been added. These features create the right ambience for a stage event, whether it’s a play, an opera, or a rock concert.
To accomplish successful lighting design, many various types of lighting instruments often are used, all of which necessitate different amounts of lighting control. While earlier lighting designs still required human intervention for control purposes, today’s light shows with their vast numbers of fixtures and associated features make stage lighting without electronic controls unthinkable.
Stage lighting and special-effect applications use long-haul data transmission networks of up to 1200 m length to provide communication between hundreds of network nodes that control lighting instruments, foggers, and other special-effect equipment. Originally developed in 1986 by the engineering commission of the United States Institute for Theatre Technology (USITT), DMX512 was the first standard ensuring reliable intercommunication for these applications.
The Entertainment Services and Technology Association (ESTA) took over the standard’s maintenance in 1998. A first revision approved by the American National Standards Institute (ANSI) in 2004 was followed by a second revision in 2008, which became the official version known as ANSI 1.11-2008, titled “Entertainment technology – USITT DMX512-A – Asynchronous Serial Data Transmission Standard for Controlling Lighting Equipment and Accessories,” or DMX512-A in short.
Topology
A DMX512 network utilizes a multi-drop topology where a single transmitter in the lighting console (master node) transmits repetitive data to multiple receivers (slave nodes). All nodes are connected through daisy-chaining, which is the output of a leading node that connects to the input of a following node (Fig. 1).
The beginning of the chain is presented by the master node in the lighting console, which provides only an output connector that connects to the input of the first slave. The end of the chain is presented by a termination resistor of 100 Ω to 120 Ω connecting to the output of the last slave. To distinguish between input and output ports during the installation process, input ports use male and output ports use female XLR-5 connectors.
Protocol
A DMX controller transmits packets of asynchronous serial data at 250 kbits/s (Fig. 2). A data packet consists of 513 time slots, 512 of which are used to transport control data. One slot, time slot 0, transmits the DMX512 identifier. Time slot 0 is also part of the reset sequence, which starts with a break (logic low) followed by a mark (logic high), also known as mark-after-break (MAB).
The subsequent time slot 0 is followed by a mark-between-time slots, here shown as Idle. Each time slot begins with a start bit, followed by eight data bits and ending with two stop bits. All data bits in time slot 0 are logic low, representing the start code 0x00, to identify the remainder of the data stream as a DMX512 packet.
The data fields of time slots 1 to 512 contain control values for the dedicated control functions of lighting instruments. For example, one of the most versatile and most commonly applied light fixtures in stage lighting is the moving head. This fully automated fixture comes with an arsenal of control features, each requiring the allocation of a dedicated DMX512 control channel (time slot). Figure 3 shows the control features for a specific moving head model. Other models might have as little as two or as many as 30 control channels.
It becomes obvious that the more complex the control features of a light fixture, the lesser fixtures can be connected to a DMX data link, also called DMX universe. Once all 512 control channels of a data link are assigned to a specific set of fixtures, a new link or universe must be started to serve other lighting instruments.
Due to the continuous increase in light fixtures per show and their rising complexity of control features, the necessity rose to structure the control and management of light fixtures more efficiently. Therefore, in 2006 a protocol enhancement to DMX512, known as remote device management or RDM, was introduced.
This protocol allows bidirectional communication between the lighting or system controller and any RDM-compliant fixtures over a standard DMX universe. RDM enables the transmission of control data to configure RDM-compliant devices, as well as the reception of status information from these devices.
To distinguish between DMX and RDM packets in a mixed-application network, the start code in time slot 0 of an RDM packet is 0xCC. Thus, DMX-compliant devices expecting a 0x00 start code ignore any RDM traffic on the same line, and vice versa, RDM devices won’t respond to data packets beginning with a DMX start code. For detailed information on RDM, refer to the official standard ANSI/ESTA E1.0, Entertainment Technology – Remote Device Management over USITT DMX512.
Physical Layer
With the introduction of RDM, responders temporarily take on the role of bus drivers when sending status information back to the lighting desk. Now data traffic is bidirectional and occurs in half-duplex mode. In this case, termination resistors at both cable ends are required.
The maximum cable length is limited to 1200 m. This is due to the fact that at this length the cable resistance of a 120-Ω, AWG-24 twisted pair approaches the value of the termination resistor, reducing the signal swing by half (6 dB). Cable recommendations include true EIA-485 cable with a characteristic impedance of Zo = 120 Ω and low-cost CAT-5 cable with Zo = 100 Ω. While it is common to match the value of the termination resistors with the cable impedance, practical results show that slight over-termination can be beneficial to signal integrity.
A big advantage over regular EIA-485 is that DMX512 specifies the use of isolated responders, preventing the design of unintended ground loops and also removing the ±7-V ground-potential-difference (GPD) limit specified in EIA-485. DMX also requires the transmitter at the lighting desk to be grounded as well as the use of a bus ground wire connecting to all responder nodes. Thus, a DMX universe utilizes the optimum grounding technique for long-haul networks, known as a single-ground-reference design.
Legacy responder designs use opto-couplers for isolation. However, LED fatigue (typically occurring after eight years of lifetime) and moisture absorption in the dielectric (typically mold compound) are causing the isolator performance to become unreliable. Removing this long-term reliability issue by using silicon-dioxide (SiO2) as isolation dielectric, digital capacitive isolators are largely replacing opto-couplers in DMX512 and other data transmission networks. SiO2 is one of the hardest materials known to man with little to no moisture absorption, so it provides high long-term stability and a minimum life expectancy of 28 years.
Figure 4 shows the input stage of a typical node design. The circuit consists of an EIA-485 transceiver with reduced slew rate for improved electromagnetic interference (EMI) performance and ±70-V standoff voltage that exceeds the specified 42 V dc of DMX512. A four-channel digital isolator connects the transceiver to an ultra-low-power microcontroller whose peripheral interfaces provide access to either stepper motor drivers (in the case of moving heads), LED drivers (from high-power LED fixtures), or analog-to-digital (ADC) and digital-to-analog converters (DAC) for sensors and actuators of other lighting fixtures.
By combining the driver- and receiver-enable pins to a single direction signal (DIR) and pulling them down via RPD, the responder is set to DMX512 mode by default. To switch to RDM mode, a simple software change in the controller is needed to toggle DIR according to RDM commands received.
Conclusion
Automated stage lighting requires the shift from purely DMX to DMX-enhanced, or RDM. The rapidly increasing numbers in lighting fixtures and control features call for the transport of large amount of data, suggesting the implementation of Ethernet into stage lighting applications.