Wired Protocols in Home Automation: KNX TP, CAN, RS-485 and Ethernet

Introduction

Wired protocols remain the benchmark for home automation installations where maximum reliability is required. Unlike wireless technologies, the transmission medium is a physical cable, which significantly reduces sensitivity to interference and enables more predictable communication.

Most of these technologies also allow power to be supplied to devices through the same cable, simplifying installation and reducing the number of power supplies required.

However, not all wired protocols are the same. Some are designed as shared buses, where multiple devices communicate over the same cable, while others use point-to-point links and require additional equipment such as switches.

Bus-based protocols

Bus-based media allow multiple devices to be connected to a single shared cable without the need for intermediate switches. They typically use a single twisted pair with a controlled impedance of around 100–120 ohms to minimize reflections and ensure signal integrity.

Their main advantage is that they require less cabling and make it easier to connect a large number of devices.

KNX TP

KNX TP (Twisted Pair) is the traditional wired physical medium of the KNX ecosystem and was specifically designed for building and residential automation applications.

Its speed is 9.6 kbit/s, which allows approximately 50 telegrams per second to be transmitted. Although this may seem low compared to modern technologies, it is more than sufficient for the vast majority of home automation applications, where events are relatively infrequent.

The main advantage of KNX TP is the enormous flexibility it offers in terms of topology. Thanks to its low transmission speed, it supports daisy-chain, star, tree, or combinations of all of them. In practice, the cable can be deployed very similarly to a conventional electrical installation.

For example, it is common to run a main cable through a hallway and branch off into distribution boxes or switches without following a strict topology.

Each line currently supports up to 256 devices and approximately 1,000 meters of cabling. There are also limits of around 350 meters between a device and the power supply, as well as between any two devices on the line. When these limits are insufficient, the installation can be segmented using line couplers or by using an Ethernet network as a backbone.

Another remarkable aspect is its robustness. The combination of low speed, tolerant topology, and decades of evolution makes KNX TP an extremely reliable technology.

Its main drawback is that it is not IP-based. This means that, in practice, it can only be used within the KNX ecosystem. In addition, installation configuration requires specific paid tools such as ETS.

CAN

CAN (Controller Area Network) is a protocol originally developed for the automotive industry to interconnect control units inside vehicles.

Although it can technically be used in home automation, it was not specifically designed for this purpose and presents some limitations when used as a general-purpose home automation network.

One of its main characteristics is flexibility. The standard allows different transmission speeds and leaves many parameters open to the specific implementation.

There are currently several generations:

• CAN 2.0: up to 1 Mbit/s
• CAN FD: up to approximately 8 Mbit/s
• CAN XL: up to 20 Mbit/s

However, the standard does not define a specific operating speed, maximum distance, or maximum number of devices. All these parameters depend on how each manufacturer implements its solution.

Unlike KNX TP, CAN is primarily designed for linear or daisy-chain topologies. Star or tree configurations are not recommended and, although they may work, they usually require significantly reduced speeds and shorter distances.

This lack of standardization makes CAN less suitable as an open home automation network. Its use is usually limited to closed ecosystems from a single manufacturer, where both the hardware and the upper-layer protocol are fully controlled.

A well-known example is Loxone. Its Loxone Link protocol uses CAN at 125 kbit/s, with a strictly linear topology, a maximum of 30 devices, and up to 500 meters of cabling. On the other hand, Loxone Tree reduces the speed to 50 kbit/s, allowing up to 50 devices and supporting star or tree topologies similar to KNX.

RS-485

RS-485 has industrial origins and remains one of the most widely used technologies in control systems, HVAC, energy, and industrial automation.

Unlike CAN, RS-485 does not even define the message format. The standard only specifies the physical transmission layer, leaving aspects such as addressing, error detection, and the application protocol completely open.

This flexibility allows for a wide range of speeds, typically between 9.6 kbit/s and several megabits per second. The maximum achievable speed depends on factors such as distance, topology, and the number of connected devices.

This freedom is also one of its main drawbacks in terms of interoperability. Two RS-485 devices are not necessarily compatible with each other, since they may use completely different protocols.

The most common protocols found running over RS-485 are industrial higher-level protocols such as Modbus RTU or BACnet MS/TP, although many manufacturers also develop proprietary solutions.

For this reason, RS-485 is usually used to integrate specific equipment such as HVAC systems, variable frequency drives, photovoltaic systems, or industrial equipment, but it is rarely used as the main home automation network in a residential installation.

10BASE-T1S

10BASE-T1S is a relatively recent addition to the world of industrial and automotive communications. It is part of what is known as Single Pair Ethernet (SPE), a new family of Ethernet standards that use a single twisted pair instead of the four pairs used in conventional Ethernet.

Its goal is to provide an Ethernet- and IP-based alternative to replace traditional protocols such as CAN or RS-485.

Unlike these, it is a much less ambiguous standard. The speed is fixed at 10 Mbit/s, and all devices share the same standard Ethernet technology, allowing full interoperability at the IP level.

The initial version of the standard supported networks of up to 25 meters and 8 devices, although more recent versions have expanded these limits to 50 meters and 16 devices. In controlled environments, higher numbers have been tested.

Its main drawback for home automation applications is topology. Due to its high transmission speed, it requires a much stricter installation than KNX TP. The network must be deployed in a linear fashion, with extremely short branches, generally under 10 centimeters.

This makes installation in a conventional home much more difficult, where star and tree topologies are usually more natural.

For this reason, it currently seems better suited for automotive applications, machinery, or internal wiring inside electrical panels rather than a distributed home automation installation across an entire house.

Point-to-point protocols

Ethernet

Ethernet is the most widely used networking protocol in the world and forms the basis of virtually all modern IP networks.

Unlike shared buses, conventional Ethernet operates using point-to-point links over four twisted pairs. This means that each device requires a dedicated connection to a switch, which determines the installation topology.

The most recommended topology is a star, with all devices connected to a central switch. Tree topologies can also be built using additional switches, although this increases both cost and installation complexity.

Its main advantage is speed. Common link speeds today include:

Speed	IEEE Standard	Name
10 Mbit/s	IEEE 802.3i	10BASE-T
100 Mbit/s	IEEE 802.3u	100BASE-TX (Fast Ethernet)
1 Gbit/s	IEEE 802.3ab	1000BASE-T (Gigabit Ethernet)
2.5 Gbit/s	IEEE 802.3bz	2.5GBASE-T
5 Gbit/s	IEEE 802.3bz	5GBASE-T
10 Gbit/s	IEEE 802.3an	10GBASE-T

Devices operating at different speeds are usually compatible with each other, automatically negotiating the highest speed supported by both ends.

Another major advantage of Ethernet is that it is IP-based. This allows a huge variety of application protocols to coexist over the same infrastructure, as long as they are based on TCP/IP or UDP/IP. These include:

• Home automation protocols such as Matter, KNX IoT, or KNX IP
• Industrial protocols such as Modbus TCP or BACnet/IP
• General network protocols and services such as HTTP or MQTT

It also allows devices to be powered via Power over Ethernet (PoE), often eliminating the need for additional power supplies.

The downside is usually cost. In addition to the cabling itself, switches must be installed at all branching points, requiring electrical power and consuming energy continuously.

For this reason, Ethernet is excellent for devices that genuinely need to handle large amounts of data, such as surveillance cameras, video intercoms, multimedia systems, Wi-Fi access points, or home automation servers. However, it may be excessive to use switch ports for simple tasks such as controlling a few relays or reading a handful of sensors.

Comparison of wired protocols

Protocol	Speed	Topology	Maximum distance	Devices per segment	IP-based	Power on same cable	Installation flexibility	Typical use
KNX TP	9.6 kbps	Bus, daisy-chain, star, or tree	1000 m	256 (previously 64)	No	Yes	Very high	Home automation
CAN	Up to 1–20 Mbps depending on version	Linear bus (star/tree only at low speeds)	Not specified, implementation-dependent	Not specified, implementation-dependent	No	No	Low	Automotive
RS-485	Up to 10 Mbps	Linear bus (star/tree only at low speeds)	Not specified, implementation-dependent	Not specified, implementation-dependent	No	No	Low	Industrial
10BASE-T1S	10 Mbps	Strict linear bus	50 m (previously 25 m)	16 (previously 8)	Yes	Yes	Very low	Automotive
Ethernet	10 Mbps to 10 Gbps and beyond	Point-to-point (star or tree using switches)	100 m per link	2 (or virtually unlimited with switches)	Yes	Yes (PoE)	High (with switches)	Building backbone networks

Conclusions

At DomoNexo, we strongly favor wired technologies, especially IP-based ones, whenever it makes sense to use them.

For applications that genuinely need to transmit large amounts of data, conventional Ethernet is an excellent solution. Surveillance cameras, video intercoms, multimedia systems, Wi-Fi access points, or home automation servers are clear examples where Ethernet provides obvious advantages.

However, using 100 Mbit/s or even 1 Gbit/s links to control a few lights or operate a blind can be excessive. It is a perfectly valid solution, but it can hardly be considered efficient.

In these cases, KNX TP remains one of the most solid alternatives currently available. Its main drawback is that it is not IP-based, which forces you to work within the KNX ecosystem and use specific tools such as ETS.

From our point of view, there is still an intermediate space that has not yet been fully covered: a wired IP-based technology with lower speeds and enough topological flexibility to be deployed throughout an entire home. Such a protocol would be the ideal physical medium for higher-level application protocols such as Matter or KNX IoT, avoiding dependency on a specific ecosystem. Initiatives such as 10BASE-T1S move in this direction, but they are still more oriented toward the automotive world than residential home automation.

For now, the industry continues to rely mainly on KNX TP for wired applications and on Thread as the IP-based wireless alternative for this type of use case.