Lighting- control systems - Intelligent Buildings and Building Automation - page 215

drive true (+) respectively. The fourth and fifth pins provide the secondary
data link, which is optional, similarly as the drive complement (-) and the
dive true (+) respectively.
The cabling specification was removed from DMX512-A after 2004,
although it is essential. It is now specified by separate standards. DMX512-A
uses the concept that data is sent in packets. Each DMX512-A data link
transmits a start code that identifies the data type and up to 512 8-bit values,
between 0 and 255, so one cable typically controls 512 dimmers or devices.
In cases where more than 512 control channels are needed, multiple DMX
universes can be used.
DMX512-A data is sent using EIA-485 voltage levels and cabling practices.
Data is transmitted serially at 250 Kbps and grouped into packets of up to
(but not necessarily) 513 bytes each. A full packet takes approximately 23 ms
to send. That corresponds to a refresh rate of about 44 Hz if the maximum
number of 512 channels is used. If a higher refresh rate is needed, fewer
channels should be used.
The DMX512-A uses 8 bits (256 levels) for each channel. But it does not
specify the correlation between the lighting level and the data (bits). Any
correlation relating the data and lighting levels is carried in the dimmers.
DMX512-A's popularity is partly due to its robustness. The cable can be
abused without any loss of function in ways that would render Ethernet or
other high-speed data cables useless.
It is worth noticing that RS-485 is used as the Physical Layer underlying
many standard and proprietary automation protocols used to implement
industrial control systems, such as Modbus and PROFIBUS, as well as
DXM512-A. RS-485 only specifies electrical characteristics of the driver
and the receiver. It does not specify or recommend any data protocol. RS-
485 enables the configuration of inexpensive local networks and multidrop
communications links. It offers high data transmission speeds (35 Mbps up
to 10 m and 100 Kbps at 1200 m). Since it uses a differential balanced line
over twisted-pair cable (like EIA-422), it can span relatively large distances
(up to 1200 m).
10.3.2 DALI - Digital Addressable Lighting Interface
It has been an aim, for which effort has been expended, that within a lighting
system every light is separately controllable but only a single control cable
is sufficient for all devices in the system. The LAN technology discussed in
Chapter 5, such as LonTalk, EBI and BACnet, actually made it possible to
achieve such an aim. One problem is that such a system may have a high cost
per node. DALI has therefore set its ambitions low. It is not targeted to be
suitable for the control of various building services systems. Instead, it seeks
to be the optimum method (in terms of cost and capacity) for controlling
lights within a large space or a number of rooms in commercial, architectural
lighting applications.