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The future of Solid-State Lighting network control protocols

issue 33 Oct / Nov 2006

Solid-State Lighting continues to dominate modern lighting applications with LED technologies improving at such a pace that even the most efficient conventional lighting sources will soon be surpassed. These new light sources offer unprecedented lighting functionality but how will the Solid-State Lighting network of the future be controlled?

This last year has shown how important LED light sources are to the future of the lighting industry with the major lighting manufacturers positioning themselves to exploit the technology in the next decade. Lighting companies such as Philips finally took full control of LED specialists Lumileds whilst GE Lighting acquired the remainder of GELCORE paving the way for LED lighting to become the main lighting technology of the future.
The advantages of LED lighting fixtures are significant (as discussed in the December 2005 LED column) however this article discusses how the future of Solid-State Lighting networks may be controlled and describes a selection of the most popular lighting control protocols used for LED fixtures.

Analogue Voltage Control (0 – 10V)
One of the earliest and simplest electronic lighting control signalling systems was the development of an analogue voltage control input to control the intensity of a light source using a DC voltage that varies between zero and ten volts. The controlled lighting would scale its output accordingly so at the highest voltage, the controlled light should be at 100% of its potential output; at 5V, it should be at 50% output; and at 0V it should be 0% output i.e. “off”.
The advantages of such a control technique are that it is simple to implement and dimming control can be via a remote potentiometer or 0-10V making it straight forward to understand, install and diagnose. The low voltage control signal means there is no special wiring constraints and therefore lighting fixtures can be connected using standard cabling.
Unfortunately, the control inputs require one wire per control channel so for an RGB LED system there would need to be three control wires per fixture. This in itself is not an issue but as the lighting installation becomes more sophisticated there may be hundreds of networked lighting fixtures and that would require several hundreds of control wires, thereby making diagnoses of problems difficult and increasing installation costs.
A further disadvantage of the Analogue Voltage Control technique include the issue of voltage drop across long control cables which could mean the signal received by the controlled light is not at the same level as the signal that was sent resulting in a reduced dimming range. In addition, analogue voltages are prone to noise and earth loops if not wired properly over long distances.
Although many LED fixture controllers still implement the Analogue Voltage Control technique its use is diminishing as more sophisticated control protocols become available.

Digital MultipleX (DMX)
Today, the DMX protocol has become the “defacto” standard in the LED lighting industry with the majority of LED driver manufacturers such as Dialight, Artistic Licence, Tryka LED, TIR Systems and Color Kinetics all incorporating the DMX protocol for controlling LED fixtures.
DMX was originally intended as a replacement for 0-10V lighting control for use in linking lighting controllers and dimmers within the entertainment industry and is a communications protocol originally developed by the Engineering Commission of USITT (U.S. Institute of Theatre Technology) in 1986 based upon a standard RS-485 architecture.
The standard has continuously evolved through subsequent revisions with the latest standard, known officially as “Entertainment Technology - USITT DMX512-A - Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories”, being approved by ANSI in November, 2004. This current standard is also known as “E1.11, USITT DMX512-A”, or just “DMX512-A”, and is maintained by ESTA.
DMX512’s popularity can be attributed to several endearing factors:
• Based upon a standard EIA485 interface;
• It is simple to implement;
• Very reliable;
• Enables control of multiple fixture networks using simple three wire control;
• Only low cost components are required.
A complex network of LED fixtures can be controlled using the DMX protocol which transmits a stream of data between the data transmitter (usually the controller) and a data reciever (usually the lighting fixture). A single DMX port, outputting this stream, can pass magnitude value information for a maximum 512 channels however multiple DMX ports can be configured to expand the number of channels available to control additional lighting fixtures.
Therefore, it is possible to control a significantly large number of channels and hence LED fixtures by simply adding additional DMX ports as shown in Table 1

A typical DMX network is shown in Figure 1 where a DMX512 controller is connected to a network of lighting fixtures in a daisy-chain arrangement. Each lighting fixture has a DMX in and generally a DMX out connector and the DMX out on the controller is linked via a DMX512 cable to the DMX in on the first fixture. A second cable then links the DMX out on the first fixture to the next device, and so on. In general, the final, empty, DMX out connector should have a DMX512 terminating plug attached into it, which is simply a 120 ohm resistor joining pins 2 and 3 of the connector however many modern fixtures negate this requirement as they are capable of auto-terminating the network.
The IEA485 specification only supports “daisy-chain” networking with up to 32 “unit loads” or fixtures on each network segment which can be up to 1000m. Practically, the use of repeaters or splitters within each network segment should be considered long before cable runs or the number of fixtures limit is reached.
The connectors themselves are usually five-pin XLR types, however within the DMX512-A standard Cat5 cabling is also an acceptable wiring topology when used in a permanent lighting installation.
One advantage of DMX is the channels can be easily assigned to a particular lighting fixture providing the functionality of each channel is mapped to the device. For example, the first LED lighting fixture on DMX port 1 in Figure 1 may require 4 channels to individually control a Red, Green, Blue and Amber bank of LEDs. In another instance lighting fixture 2 may just require three channels for R,G,B control whilst LED fixture 3 might require one channel to provide dimming of a white LED fixture.
At the heart of the DMX protocol is the DMX512 packet (shown in figure 2) which contains all of the information required to control fixtures within a DMX network. The “Start Code” within a DMX packet is the first byte of information after the break and is used as a flag to indicate the type of data that follows. A value of ‘0’ indicates that the following frames contain lighting fixture intensity level information. The other 255 codes are not defined in the DMX specification however they are utilised within the new Remote Device Management (RDM) standard (see next section) that is based upon DMX.
Each DMX port transmits up to 512 eight-bit channel values, between 0 and 255 and a full DMX packet takes approximately 23 mS to send across the lighting network which corresponds to a refresh rate of about 44 times per second. This refresh rate is generally considered appropriate for the majority of applications and is above the perception rate of the eye.
The only significant disadvantage of the DMX control protocol is that it is uni-directional in nature meaning that the information is transferred one way from the DMX controller to the lighting fixture(s). Therefore, it is impossible to monitor the performance of any lighting fixtures within a DMX network and for example the DMX controller could not monitor whether a lighting fixture had failed.
Despite being uni-directional, the DMX protocol has become the “defacto” standard for LED fixtures, however it is clear that for future LED control protocols a bi-direction standard that allows automatic lighting fixture address assignment and performance management will be essential due to the significant advantages that LED fixtures offer the lighting designer and installer.

Bi-directional control protocols
There are a number of bi-directional protocols being developed, however none of them have taken a significant market share as it is still very early days and many lighting manufacturers are only just begining to launch new products. For example, there is the Digital Addressable Lighting Interface (DALI) protocol used primerily in fluorescent lamp ballasts, the Remote Device Management (RDM) which is an enhancement to DMX that allows bi-directional communications and not forgetting control protocols based upon TCP/IP that are commonly found in most computer networks including the Internet.

Remote Device Management (RDM)
Remote Device Management is a draft protocol enhancement that will allow bi-directional communication between a lighting or system controller and attached RDM compliant devices over a standard DMX line. This protocol will allow configuration, status monitoring, and management of these devices in such a way that does not disturb the normal operation of standard DMX devices that do not recognise the RDM protocol. RDM provides a significant advantage over competing lighting protocols as it is backwards compatible with DMX devices thus enabling lighting designers, installers, rental companies and manufacturers to continue to utilise the significant investment they have made in LED lighting fixtures so far.
The RDM standard was developed by the ESTA Technical Standards and is officially known as “ANSI/ESTA E1.20, Entertainment Technology - Remote Device Management over USITT DMX512”. Copies of the standard may be downloaded from the ESTA website.
Since the RDM protocol is transparent within the DMX512 protocol it provides full compatibility with DMX lighting networks and companies such as Radiant Research, Artistic Licence and Wybron are developing LED lighting controllers, drivers and test equipment to enable lighting designers to develop intelligent networks.
RDM will use the time between packets to transmit data to supported devices thus not adding any data that standard DMX devices would consider valid level information. All RDM devices on the link will have a unique identifier (UID) that will consist of a manufacturer ID and serial number; the controlling device can then use a “discovery” procedure to acquire the UIDs of all connected devices and would then be able to communicate automatically to a specific device or in groups identified by the manufacturer.
RDM compliant lighting networks offers the following benefits:
• Ability for the console to set the base address of the fixture. There will no longer be a need for technicians or installers to set the DMX address using DIP switches;
• RDM devices can be firmware upgraded via the DMX512 signal;
• By allowing bi-directional communication, it will be much easier to mix DMX installations with sophisticated Ethernet protocols such as ACN;
• Control of individual units (individual addressing) or groups (group addressing) is possible;
• A simultaneous control of all units is possible;
• No interference of data communication is to be expected due to the simple data structure;
• Control device status messages (lamp fault, ....), (report options: all/by group/by unit);
• Automatic search of control devices;
• Simple formation of groups of lighting fixtures;
• Automatic dimming of all units when selecting a scene;
• System with assigned intelligence (every unit contains amongst other things the following data: individual address, group assignment, lighting scene values, fading time...);
• Operational tolerances of LEDs can be stored as default values (for example for the purpose of energy savings maximum values can be set);
• Fading: adjustment of dimming speed;
• Identification of unit type;
• Lower system cost and more functions compared to 1–10V systems.
The RDM protocol allows the controller to obtain feedback information such as if the LED light is overheating, operating hours, what the LED current draw is, what the mains or DC voltage is and it can automatically identify the fixtures optimum operating conditions.
In the future the RDM protocol will change the way that Lighting Technicians set up and maintain their lighting networks making installations far simpler and much quicker saving time and money. The RDM protocol is still being refined, however RDM compliant products such as the iDrive™ from Radiant Research Limited shown in figure 3 are already in production. All RDM compliant devices means that when the RDM standard is finalised, the products can be upgraded if required.
It is envisaged that RDM lighting networks will become a natural extension of DMX-enabled LED lighting fixtures as it offers:
• Backward compatibility with DMX the current “defacto” standard;
• A low cost route to bi-directional control protocol;
• Easy installation and configuration;
• A powerful set of control features that utilise the advantages of LED systems.

Digital Addressable Lighting Interface (DALI)
DALI was established as a successor for the Analogue Voltage Control (1-10v) protocol and is an open standard as defined by the International Electrotechnical Commission (IEC) 60929, standard for fluorescent lamp ballasts.
The DALI communication protocol is designed to operate on a two-wire cable, and the range includes a series of controllers, lamp interface units and special purpose-designed modules, featuring converters and other products to ensure maximum system versatility from a variety of manufacturers including Osram, Philips, Helvar and Tridonic Atco. Using a bi-directional data exchange, a DALI controller can query and set the status of each lighting unit within a DALI network.
The advantages of DALI systems include many of the bi-directional functions outlined in the RDM protocol. However there are some drawbacks to implementing DALI, for example, it can only be operated with a maximum of 64 lighting fixtures due to a 6-bit address structure and it is not compatible with DMX thus requiring LED lighting manufacturers to lose their investment in current products.
Although DALI has been in the market place for quite some time it has not been widely adopted due to the premium cost attributed to a DALI controlled lighting fixture as compared to a standard (non-DALI) fixture.

IP based networks
Today the Internet uses IP protocols as its core communications layer to transfer information between one device and another. However it is already starting to attract the attention of lighting organisations as a network for communicating with lighting fixtures and controlling more complex devices like video playback servers.
The advantages of using TCP/IP include:
• A low cost infrastructure - maximum opportunity to use off-the-shelf technology;
• Scalability - Near unlimited lighting networks are possible;
• Compatible with Network and Internet protocols so easy to control lighting networks remotely;
• Ease of configuration - networks should be easily configured;
• Control protocol speeds are very high (now Gibabyte ethernet);
• Fault tolerance - the system needs to be fault tolerant if packets of information are lost.
ESTA is developing a networking protocol called Architecture for Control Networks (E1.17) [ACN] that proposes a way forward for controlling lighting fixtures across IP networks.
Large scale deployment of ACN-based lighting networks is not yet being seen in the market place predominantly because most LED lighting fixtures utilise the simpler and less expensive DMX standard. However, if the cost of implementing IP-based microprocessors within lighting fixtures and LED drivers can reduce close to that of an 8-bit PIC controller commonly found in most LED fixtures today then perhaps IP-based lighting will become commonplace.

Today, DMX is the prefered control protocol for LED lighting fixtures. However, the need for more control over the fixture performance will necessitate that a bi-directional protocol will become the next evolution for LED fixture control. There are several contenders that could easily become the next protocol standard for LED fixture controls, however the most likely to succeed today is RDM as it provides an excellent upgrade path for DMX systems enabling current LED manufacturers to retain their R&D and capital investment whilst offering significant advantages to the lighting designer and installation engineer.
The business case for current LED fixture manufacturers to convert their DMX products over to RDM compliant devices is straightforward and most DMX enabled products only need minor hardware and software modifications necessitating the need to invest in substantial R&D expenditure.
Therefore I predict that RDM will, in a short space of time, enable LED fixtures to become truly intelligent by providing constant feedback to lighting network controllers enabling the system to fully monitor LED fixture performance and perhaps in the future enable systems to detect faults before they become apparent to users. Then again, I could be completely wrong!


table 1

Table 1: Channels can be expanded by adding new DMX ports


Related Articles
  • Figure 1

    Figure 1: A typical DMX network controlling three DMX ports each containing a series of LED lighting fixtures


  • Figure 2: A diagram of a typical DMX Packet at the heart of the DMX standard


  • Figure 3: The new iDrive LED fixture driver from Radiant Research Ltd that is fully RDM compliant



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