What is the Horti Lighting Protocol (HLP)?
A publicly available standard from Stichting Hortivation that lets a climate computer control LED grow lights of any brand in the same way. What it contains, how it works and what to watch out for.
One language for every light
The move to LED brought many new fixtures and suppliers, and almost every supplier had its own way of controlling them. Every link between climate computer and lights became custom work, with higher costs for installation and support.
Three climate computer suppliers therefore drew up one communication protocol together with lighting suppliers. Stichting Hortivation in Naaldwijk, the Netherlands, manages it and publishes the specification on its website. If your lighting system has a gateway that speaks HLP, any automation system that supports HLP can work with it.
Versions
- 0.9 · April 2023draft for comments from lighting suppliers
- 1.0 · May 2023first final version
- 1.0.1 to 1.0.3 · June 2023 to March 2024corrections and clarifications
- 1.1.0 · 26 May 2025new modules Universal Outputs and Preferred Setpoints
From climate computer to fixture
The chain is short: the automation system talks to the gateway of the lighting system, and the gateway takes care of the rest of the lighting network.
Animation of the HLP chain. The automation system, as Modbus master, writes setpoints to the lighting gateway. The gateway passes them on to two light groups, whose red, blue, far-red and white channels change. The automation system then reads back from the gateway what the lights are actually doing.
Writing: the automation system sets a setpoint per light group and per channel (Modbus function code 16).
- Writing: the automation system sets a setpoint per light group and per channel (Modbus function code 16).
- The gateway passes the setpoints on. The lights must reach them within 20 seconds.
- Reading back: the automation system reads realised light, power and status (function code 3).
The technology in brief
| Transport | Standard Modbus: Modbus TCP over Ethernet, or Modbus RTU over RS-485 (half duplex). With many light groups, TCP is preferred. |
|---|---|
| Roles | The automation system is the Modbus master, the lighting system the Modbus slave. An automation system is expected to control several lighting systems independently at the same time. |
| Registers | 16-bit integer holding registers, big-endian. Reading with function code 3, writing with function code 16. |
| Modbus TCP | Port 502; the gateway has a static IP address and a documented way to return to factory settings. |
| Scale | Up to 100 light groups (control areas), each with up to 8 channels. That is the limit of the protocol; a lighting system may support fewer. |
Per group, per colour
A light group, a control area in the standard, is a group of lights with the same settings. Each channel within such a group stands for exactly one colour or one spectrum, for example red, blue, far red or white. Two channels for the same colour are not allowed.
There are two ways to control it. Percentages are the simplest: the standard assumes the percentage is linear with the light output. If you want to control the spectrum precisely, you work in µmol/s or µmol/s/m² with the Exact Light Control module.
If a fixture receives no setpoint from the gateway after starting up, it falls back to a predefined power-on light level (power-on setpoint).
A base and six modules
The base is only setpoints per light group per channel. A lighting system can support modules on top; if it supports a module, it must support all parts of it.
Base
Setpoints per light group and per channel, in percent.
System status
Feedback on the status of the lighting system, such as faults.
Power
The actual power used by the fixtures.
Exact Light Control
Control the exact light output per spectrum in µmol/s or µmol/s/m², with a check beforehand.
Universal Inputs
Up to ten freely usable values per light group from the lighting system to the automation system.
Universal Outputs
Up to ten freely usable values per light group in the other direction. New in 1.1.0.
Preferred Setpoints
The lighting system proposes a spectral composition while the automation system keeps control of the power balance. New in 1.1.0.
What the FairGrow HLP node supports today: the base, System status and Power. Exact Light Control and Preferred Setpoints will follow. HLP node
What the standard pins down
The standard fixes a few hard times. They decide how quickly a rule becomes visible in the greenhouse and how long you wait after a power cut.
Logarithmic scale, from milliseconds to minutes
What the standard does not cover
HLP does not prescribe any security. It is plain Modbus, without password or encryption. The standard therefore says itself that the network between automation system and lighting gateway must not be connected to the internet, nor to a network that is.
For you this means: keep the lighting network separate. If you still want to see light and energy data in an online platform, you need a device that sits on both networks but passes nothing from one to the other.
How the FairGrow HLP node does that →What HLP gives you
Choice without lock-in
Lights and controls no longer have to come from the same supplier, as long as the gateway speaks HLP.
Data that looks the same everywhere
Realised light, power per light group and faults sit in fixed registers. Anyone reading along does not need a separate integration per lighting brand.
A fallback inside the light
The power-on light level is stored in the lighting system. If control drops out, the fixture knows what to do.
Room to grow
Start with the base and add modules when your installation supports them, such as exact light control per spectrum.
About the standard
Where do I find the specification?
On the website of Stichting Hortivation, under data. The current version, 1.1.0 of 26 May 2025, is available there as a download. Copyright belongs to Hortivation.
How do I know whether my lights support HLP?
Ask your lighting supplier whether the gateway of your lighting system speaks HLP, and which version and modules. The modules decide what you can read and control.
What is the difference between control in percent and in µmol?
Percentages are the simplest and assume a linear light output per channel. With µmol/s or µmol/s/m² you control the actual light output per spectrum; the lighting system then has to support the Exact Light Control module.
Can I only read along, without controlling?
Yes. Reading uses Modbus function code 3 and changes nothing on your lights. That way you can follow realised light, power and faults while your climate computer stays in control.
May two systems write to the same lights at once?
We advise against it. The standard assumes one automation system as master. Two systems writing setpoints at the same time overwrite each other.
What happens after a power cut?
The gateway must be reachable again within 120 seconds and a fixture within 180 seconds. If a fixture receives no setpoint, it falls back to the configured power-on light level.
Using HLP without a climate computer?
The FairGrow HLP node measures daylight, works out the daily light integral and tops up your LED light only where it is needed, through HLP and with a lighting network that stays separated from the internet.
Source: Horti Lighting Protocol v1.1.0 (26 May 2025), Stichting Hortivation. This page summarises the standard; the specification itself is authoritative. FairGrow is not affiliated with Stichting Hortivation.