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IEC 62756-1-2015: Digital Load Side Transmission Lighting Control (DLT) – Basic Requirements
This standard defines the Digital Load Side Transmission (DLT) protocol for lighting control, an innovative communication technology that transmits digital control signals on the load side of the control gear — after the mains power has been processed. DLT enables dimming, colour control, and colour temperature control of LED and fluorescent luminaires without additional control wiring.
1. DLT Communication Principle and System Architecture
IEC 62756-1-2015 introduces DLT as a lighting control technology that operates on the load side of the control gear, fundamentally different from traditional mains-borne signaling (such as ripple control) that transmits on the supply side. In DLT, both the mains power and the digital control signal are carried on the same conductors, but the control signal is applied after the mains rectification stage within the control gear, enabling reliable communication even with LED drivers and electronic controlgear that may heavily attenuate mains-borne signals.
The system employs a master-slave architecture: the control device (master) transmits commands, and the control gear (slave) receives and executes them. Each half-cycle of the mains waveform is divided into three distinct time periods: the supply period (for power transfer), the operating period (reserved for internal control gear functions), and the data period (for digital communication). This time-division approach ensures that data transmission does not interfere with the primary power conversion function of the control gear.
| Time Period | Duration (at 50 Hz) | Function |
|---|---|---|
| Supply period | ~6.5–7.5 ms | Main power transfer to control gear |
| Operating period | ~0.5–1.5 ms | Internal control gear processing |
| Data period | ~1.0–2.0 ms | Digital command transmission |
The standard defines electrical characteristics for five nominal mains voltage ratings: 100 V, 120 V, 200 V, 230 V, and 277 V, each at 50 Hz and 60 Hz. For each voltage rating, detailed timing parameters and voltage/current levels are specified for the supply period and data period separately.
A crucial distinction from DALI: DLT operates on the mains power conductors (no extra control wiring), while DALI requires a dedicated 2-wire control bus. This makes DLT particularly advantageous for retrofit installations where running additional control cables is impractical. However, DLT communication is half-duplex and limited to one half-cycle per command, requiring careful timing synchronization between master and slave devices.
2. Telegram Structure and Protocol
The DLT protocol uses a 3-byte (24-bit) telegram structure transmitted during the data period of each half-cycle. The telegram consists of:
- Group number (5 bits): Identifies the target group of control gear (up to 32 groups)
- Telegram type (3 bits): Defines the command type (8 types, types 0–7)
- Data payload (15 bits): Carries command-specific parameters
- Parity bit (1 bit): Even parity for error detection
The eight telegram types provide comprehensive lighting control capabilities:
| Type | Name | Data Content | Application |
|---|---|---|---|
| 0 | Brightness | 11-bit dimming level (0–100 %) | Dimming, on/off control |
| 1 | Colour control | CIE (x,y) chromaticity coordinates | RGB/W colour tuning |
| 2 | Colour temperature | CCT value in Kelvin | Tunable white lighting |
| 3 | Reserved | — | Future standardization |
| 4 | Reserved | — | Future standardization |
| 5 | Commissioning | Group number assignment | Installation and setup |
| 6 | Manufacturer specific | Vendor-defined | Proprietary features |
| 7 | Extended telegram | Multi-frame data | Complex commands |
Data timing is defined at the bit level: each bit is encoded as a specific voltage level and duration within the data period. The standard defines four possible bit transition patterns (00, 01, 10, 11) for encoding two bits per transmission frame, achieving an effective data rate of approximately 100–200 bits per second (depending on mains frequency and data period duration).
Engineering Insight: The DLT protocol supports CIE 1931 chromaticity (x,y) coordinates for colour control, enabling precise colour tuning across the entire CIE colour gamut. This goes beyond simple correlated colour temperature (CCT) control by allowing designers to specify exact chromaticity points, including colours outside the black-body locus. The colour gamut of the connected luminaire must be communicated during commissioning (e.g., RGBA vs. RGBW vs. tunable white) to enable proper interpolation of requested colours.
3. Electrical Characteristics and Timing Specifications
DLT imposes strict timing requirements on the control interface. The supply period must provide sufficient energy to maintain control gear operation during the data period when the mains supply is briefly interrupted for data transmission. The standard specifies rise time and fall time of the data signal at the control interface: tr and tf must be between 1 μs and 100 μs, ensuring electromagnetic compatibility with the lighting environment.
The power-up timing is critical: upon application of mains voltage, the control device must wait a minimum of 500 ms before transmitting any telegrams to allow the control gear to complete its internal initialization. The standard defines the electrical characteristics during the off state of the control gear, including both power-controlled off state (maintained by the control device removing supply period power) and telegram-controlled off state (the control gear remains powered but in standby).
| Parameter | 100 V System | 230 V System | 277 V System |
|---|---|---|---|
| Supply period start voltage | 80 V | 184 V | 222 V |
| Data signal low voltage | ≤ 15 V | ≤ 34 V | ≤ 42 V |
| Data period duration (min) | 1.0 ms | 1.0 ms | 1.0 ms |
| Maximum data rise/fall time | 100 μs | 100 μs | 100 μs |
| Power-off hold-up time | ≥ 50 ms | ≥ 50 ms | ≥ 50 ms |
A critical design constraint: The DLT data period occurs during the zero-crossing region of the mains half-cycle. For LED drivers using valley-fill or active PFC stages, the energy storage capacitor must maintain the DC bus voltage within operating limits during the data period when mains current is interrupted. Insufficient hold-up capacitance will cause the control gear to reset or the LED output to flicker. Designers should calculate the required capacitance based on: C ≥ (2 · P · tdata) / (Vbus_min² − Vbus_off²), where P is the output power and tdata is the data period duration.
4. FAQs
Q: How does DLT differ from DALI in practical installation?
A: DLT requires no additional control wiring — it operates over the existing mains power conductors. DALI requires a dedicated 2-wire control bus. This makes DLT more cost-effective for retrofit projects but limits the maximum number of addressable devices compared to DALI (which supports up to 64 individual addresses). DLT uses group-based addressing (up to 32 groups).
Q: Can DLT and DALI coexist in the same lighting installation?
A: Yes, they can coexist as separate control systems for different zones or luminaires. However, they cannot directly communicate with each other without a gateway/bridge device. Some manufacturers offer hybrid control gear with both DLT and DALI interfaces.
Q: What is the maximum cable length for DLT communication?
A: The standard does not explicitly specify a maximum cable length since DLT operates over the mains power conductors. In practice, the communication range is limited by the mains wiring impedance and voltage drop. For typical 1.5 mm² or 2.5 mm² copper wiring, reliable communication is achievable up to 200–300 m from the control device, subject to the voltage drop limits specified in the installation standard IEC 60364.
Q: Does DLT support emergency lighting operation?
A: The standard does not specifically address emergency lighting. For emergency lighting applications, additional requirements from relevant safety standards (e.g., IEC 62034 for self-contained emergency lighting) would apply. The telegram-controlled off state feature (Clause 6.9.3) allows the control gear to enter a low-power standby mode while maintaining the ability to respond to emergency commands.
