How DALI Works: A Detailed Technical Explanation

How DALI Works

1. DALI Communication Fundamentals

DALI (Digital Addressable Lighting Interface) operates as a master-slave digital communication protocol using a two-wire bus system that combines power and data transmission. Here’s the technical breakdown:

Electrical Specifications

  • Bus Voltage: 16V DC nominal (operating range: 9.5V – 22.4V)
  • Current: 250mA max per segment
  • Data Encoding: Manchester encoding (bi-phase)
  • Data Rate: 1,200 bits per second (baud)
  • Message Structure: 16-bit frames (2 bytes)

2. The DALI Telegram Structure

Each DALI command consists of a 16-bit message with the following components:

Bit PositionFunctionDescription
0Start BitAlways HIGH (1)
1-8Address ByteContains target address (individual/group/broadcast)
9-16Command ByteControl instruction (dimming, scene recall, etc.)
17-18Stop BitsTwo HIGH bits (11)

Forward Frame (Controller → Device):

[Start][Address][Command][Stop]

Backward Frame (Device → Controller):

[Start][Response][Stop]

3. Addressing Scheme

DALI supports three addressing modes:

  1. Short Addressing (Primary Mode)
    • 64 individual addresses (0-63)
    • 16 groups (0-15)
    • Broadcast address (all devices)
  2. Extended Addressing
    • 24-bit addressing for large systems
    • Used in DALI-2 for control devices
  3. Special Addresses
    • Emergency lighting (address 64-71)
    • Configuration addresses (80-255)

4. The Communication Process

Step 1: Bus Initialization

  • Power supply provides 16V DC to bus
  • All devices receive operating voltage
  • Bus capacitance must be <1µF for signal integrity

Step 2: Command Transmission

  1. Master sends forward frame (16 bits)
  2. Each device decodes address byte:
    • If addressed, processes command
    • Others ignore the message
  3. Addressed device executes command:
    • Dimming: 8-bit resolution (0-255 steps)
    • Scene recall: 16 preset scenes available

Step 3: Acknowledgment (Optional)

  • Some commands require backward frame
  • Device responds with 8-bit status:
    • Lamp failure
    • Power-on status
    • Limit errors

5. Physical Layer Operation

Signal Characteristics

  • HIGH: 9.5V-22.4V (logic 1)
  • LOW: 0V-4.5V (logic 0)
  • Transition: Voltage change must occur in <10µs

Manchester Encoding

  • Each bit period = 833µs (1/1200Hz)
  • Logic 1: HIGH-to-LOW transition at mid-bit
  • Logic 0: LOW-to-HIGH transition at mid-bit

Bit Representation:

1: _▔▔

0: ▔▔_

6. Power Supply Requirements

The DALI bus requires special power supplies with:

  • Current limiting: 250mA max
  • Short-circuit protection
  • Galvanic isolation (recommended)
  • Low ripple: <100mV p-p

Power-up Sequence:

  1. Bus voltage ramps to 16V in <1s
  2. Devices initialize within 100ms
  3. System ready for commands

7. Timing Considerations

ParameterValueDescription
Bit Time833µs1/1200 baud
Inter-frame Gap≥22 bit times~18.3ms
Response Time10-400msDevice-dependent
Fade Times0.1s-15minConfigurable per device

8. Error Handling Mechanisms

  1. Collision Detection
    • If multiple devices respond simultaneously
    • Master detects corrupted backward frame
    • Retries transmission after delay
  2. Signal Integrity Checks
    • Valid Manchester encoding required
    • Invalid messages discarded
  3. Device Feedback
    • Lamp failure reporting
    • Power supply monitoring
    • Thermal warnings

9. Advanced DALI-2 Features

The updated standard adds:

  • Strict timing requirements (command response within 12ms)
  • Extended command set (32 new commands)
  • Control device support (buttons, sensors)
  • Enhanced diagnostics (detailed status reporting)

10. Real-World Signal Example

A typical dimming command (Set Level to 50%):

Forward Frame:

1 00011001 01001100 11

(Start) (Addr 25) (Cmd 76) (Stop)

Backward Frame:

1 00000000 11

(Start) (Ack) (Stop)

This technical architecture enables DALI to provide reliable, precise lighting control while maintaining simple wiring infrastructure – making it the preferred choice for professional lighting systems worldwide.