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 Position | Function | Description |
|---|---|---|
| 0 | Start Bit | Always HIGH (1) |
| 1-8 | Address Byte | Contains target address (individual/group/broadcast) |
| 9-16 | Command Byte | Control instruction (dimming, scene recall, etc.) |
| 17-18 | Stop Bits | Two 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:
- Short Addressing (Primary Mode)
- 64 individual addresses (0-63)
- 16 groups (0-15)
- Broadcast address (all devices)
- Extended Addressing
- 24-bit addressing for large systems
- Used in DALI-2 for control devices
- 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
- Master sends forward frame (16 bits)
- Each device decodes address byte:
- If addressed, processes command
- Others ignore the message
- 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:
- Bus voltage ramps to 16V in <1s
- Devices initialize within 100ms
- System ready for commands
7. Timing Considerations
| Parameter | Value | Description |
|---|---|---|
| Bit Time | 833µs | 1/1200 baud |
| Inter-frame Gap | ≥22 bit times | ~18.3ms |
| Response Time | 10-400ms | Device-dependent |
| Fade Times | 0.1s-15min | Configurable per device |
8. Error Handling Mechanisms
- Collision Detection
- If multiple devices respond simultaneously
- Master detects corrupted backward frame
- Retries transmission after delay
- Signal Integrity Checks
- Valid Manchester encoding required
- Invalid messages discarded
- 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.
