DMX512, RDM, DALI Control System

DMX-512A

This standard was developed for digital data networks used to control LED lights and other devices. At the physical layer, this protocol uses EIA-485 differential signals and packet transmission. The exchange is possible only in one direction and does not provide for checking and correcting errors.

The DMX512 controller produces serial asynchronous data transfer at 250 kbaud. Data transmission over 512 channels will take about 23ms, which corresponds to a 44Hz refresh rate. For more frequent updates, the forwarding is done on fewer channels.

The widespread use of the DMX-512A protocol is due to several reasons:

  • it is based on the EIA485 interface;
  • ease of execution;
  • high reliability;
  • the ability to control multiple networks of fixtures over three wires;
  • low cost of the element base;
  • the control interface is isolated from the luminaire, i.e. protected;
  • the maximum number of devices is 512. Further expansion is only possible with additional DMX ports.

A typical DMX network structure is shown in Figure 3. The controller is connected to a line of luminaires connected in series.

The EIA485 standard is only for daisy chain systems. Each segment can have up to 32 devices, the total length of the connecting wire is 1 km. With the help of special RS485 ICs, you can increase the number of devices connected to one port. So, when using the ISO15 Texas Instruments IC, there can be up to 128 devices in one segment.

Some LED driver manufacturers state that any number of drivers can be connected to a single DMX port. This is not confirmed in the official standard. In most of these drivers, the output is restored to the same DMX signal that was received at the input. In this case, a time delay accumulates, which becomes noticeable in large installations. In addition, a failure that occurs in one driver will be repeated by all subsequent ones.

Rice. 3. Lighting system with DMX interface

The advantages of DMX include the free assignment of channels for the lamp. For example, the first light connected to the first port (see Figure 3) requires 4 channels to control red, green, blue, and amber. For the second – 3 channels (RGB). For luminaire 3, one channel may be sufficient to dim the white LED.

The DMX protocol uses packet transmission. The first byte of the information field contains the “start code”. It is used as a flag to indicate the type of data being transferred. A value of 0 indicates that the package contains information about the intensity of the light flux. The remaining 255 codes are not defined. They are used in the RDM protocol (see below).

Each DMX port transmits up to 512 8-bit channel values. It takes 23ms to transmit a full packet, which corresponds to an update rate of 44 times per second. This frequency is suitable for most applications and is not perceived by the eye.

The only drawback of the DMX protocol is the one-way transmission from the controller to the light source. Accordingly, it is impossible to monitor the status of luminaires and track failures.

DMX converters are widely used in architectural lighting where connecting lines can be too long. While DMX512 wireless networks can operate up to 1000m, most connections cannot be longer than 300-450m for reliability reasons.

Bidirectional protocols

Among the many bidirectional lighting control protocols, Digital Addressable Lighting Interface (DALI), Remote Device Management (RDM), Architecture for Control Networks (ACN) and KNX have received international recognition.

The new version of the DMX512/2000 protocol, which is in full swing, will add connector types, high-voltage protection on signal pins, and resolve problems on the ground bus. It may be possible to allow two-way transmission.

RDM

This protocol is an upgrade of the DMX512 protocol that allows you to receive data from a light source over standard DMX lines. Configuration, status monitoring, control of RDM devices, reading of key indicators (current consumption, operating temperature, operating time, mains voltage, color rendering index, etc.) are possible – and all this without interfering with the main operation of standard DMX devices that do not support RDM. The big advantage of RDM is that it is backwards compatible with DMX, allowing you to use your existing infrastructure.

The protocol was created by the ESTA organization as part of the technical standards development program. The DMX package is left unchanged. Data transfer for RDM devices is carried out in the intervals between packets. RDM devices have a unique identification number by which the controlling device recognizes them.

RDM lighting systems have the following advantages:

  • the ability to set the base address of the lamp. This speeds up the installation of fixtures and eliminates the need to assign DMX addresses manually;
  • software update via RS485 interface;
  • the ability to create DMX-systems with Ethernet support (ACN protocol, etc.);
  • management of individual devices or a group;
  • simultaneous control of all devices in the network;
  • no interference due to simple data structure;
  • transmission of status messages (for example, about failure) from one, several or all devices in the network;
  • automatic recognition of lighting fixtures;
  • a simple principle for the formation of groups of lamps;
  • automatic dimming of all devices in accordance with the selected scene;
  • elements of an intelligent system (individual address, grouping, preset scenes, fade time, etc.);
  • allowable values ​​for LEDs are set by default (for example, in order to save energy, you can set maximum values);
  • the ability to set the speed of fading;
  • device type identification;
  • lower system cost with more functionality than analog 0-10V control.
  • Despite all the advantages, the protocol is not widely used due to some disadvantages:
  • high cost of electronic circuits;
  • lack of controllers that can use the additional power of RDM devices, as a result – a high price;
  • a small number of LED drivers that support RDM.

These shortcomings have already been eliminated, so in the future this standard may become the main one for lighting systems.

DALI

The DALI standard was developed as an extension of the AVC 1-10V analog interface. It is an open standard for ballasted fluorescent lamps. At the end of 2009, the standard was extended. In particular, it became possible to control LED installations. The protocol implies data transmission over two lines (see Fig. 4).

Rice. 4. Data transmission via DALI network

Advantages of DALI:

  • control of 64 devices on one bus;
  • up to 16 variants of scenes can be formed in one space;
  • control wires are connected directly to the ballast or control circuit, making it easy to connect when there are many zones or groups;
  • non-linear dimming up to 0.1% (theoretical limit).

The dimming curve on a logarithmic scale is shown in Figure 5. Most drivers cannot dim an LED to this level because the minimum current is 5-6 mA, i.e. 1.5% for a driver rated at 350mA. It is possible to use PWM, but this is undesirable in general white light systems, because due to the small duty cycle in low light, flicker can be noticeable to the eye. In the near future, drivers with a forward current of less than 1 mA (0.3%) containing a signal processor will appear. They will provide blackout with perfect visual performance.

 

Rice. 5. Dimming Curve

The DALI standard uses Manchester code encryption for error correction. The maximum baud rate is 1200 baud. The maximum segment length is 300 m for a 1.5 mm2 cable, 100 m for a 0.5 mm2 cable and 150 m for a 0.75 mm2 cable.

As with RDM, the DALI protocol has not been widely adopted due to the high cost of the controllers. This problem has already been solved, so the protocol will be in demand in new projects.

IP systems

Many modern devices communicate over the Internet. Gradually, this technology penetrates into lighting systems. Benefits of using TCP/IP protocols:

  • low cost of infrastructure;
  • scalability – the ability to connect an almost infinite number of networks;
  • compatibility with network and Internet protocols allows you to control lighting remotely;
  • ease of configuration;
  • high transmission speed;
  • error tolerance.

Currently, the ACN network protocol is being developed – architecture for control networks (E1.17) for controlling lighting systems over an IP network. The protocol is a superset of UDP/IP. Communication is carried out over low-cost standard Ethernet or Wi-Fi lines.

The ACN protocol is fully bidirectional. Each device has a unique identification number by which the controller recognizes connected devices. In addition, a file describing all the possibilities of the light source is attached to each device. Thus, the controller will be able to control the lights that will appear in the future. To migrate from DMX512 to ACN, an intermediate DMX-over-ACN (Streaming ACN, or BSR E1.31) protocol has been developed.

 

References

  1. Archenhold G. The Right Protocol//Mondo, Issue 60, April–May 2011.
  2. Higgins D. Lighting Control Protocols//ESTA Protocol, 2000

 

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