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Practical introduction to KNX

This article is an introduction to KNX for beginners in the field of home automation and electrical installations. The main goal here is to help DIY enthusiasts to understand the basics of this building automation architecture, its cost and is it suitable for their projects.

What is KNX?

KNX is a twisted pair communication standard used in the building automation and industry. In a similar version, it exists since 1984 under the name EIB. In 1990, at the initiative of equipment manufacturers, KNX unites EHS, EIB and Bâtibus standards under one standard, and in 1999 the KNX Association was formed to develop it.

THE STRENGTHS
KNX is a well-established and mature standard, adopted by many major manufacturers of ultraterminals and automation – ABB, Siemens, Jung, Hager, Schneider Electric, etc. It is though a manufacturer-independent standard. The design of any product from any manufacturer must strictly comply with the standards specifications in order to be KNX certified, with each new product passing a series of tests. This leads to quality products that, regardless of their brand, can work in one system. There are currently over 6000 KNX products that fulfill all possible control and detection functions in a single building.

 Another strength of KNX is distributed logic – instead of a central “brain” managing everything, many smart products communicate with each other. Thus, the system remains operational when a faulty component is present.

THE WEAKNESSES?
High price of components that matches the quality and the capabilities. If you choose your products well, everything is achievable. In short: With KNX you pay hard, but there will rarely be disappointments.

 The initial investment is large. Unlike wireless systems like Zwave/zigbee, you can’t build a KNX system step-by-step. Everything is planned and done at once – from cabling to startup.

Therefore, the implementation of KNX systems is appropriate for new buildings or major overhauls that include replacement of the electrical installation.

How much costs to light a bulb with KNX

Cost of KNX installation
Even for such a simple action like lighting a bulb, you will need several KNX and traditional components.

Lets illustrate the principle of operation and order of value of the system through the following case study: How much to light a bulb with KNX?

Here is the bill of materials (design and installation not included):

  1. KNX actuator, which is basically a contactor with knx networking. It turns the lamp on and off – start at $50 if you buy more than 10.
  2. Lighting switch: KNX switch, or classic mono-stable pulse switch with KNX micromodule – the simplest KNX switches start at $50.
  3. 30V KNX power supply: it is quite specific. It supplies all KNX modules via the KNX communication cable (note, this is not a line power supply, it powers only the KNX modules – see section 5). Here prices are exploding fast – $150 for a small power supply, but rather consider $200.
  4. KNX standard communication cable (or other twisted pair) that connects all KNX devices (key, actuator and power supply) – $0.40/m
  5. Standard power line for the lamp.
  6. Traditional circuit breaker to protect the power line.

In order to have a complete picture of the costs, we should also count the software liscence price for addressing and setting up the KNX modules.

This example is of cours exagerated, because the cost of some components is shared between several devices and the final price for automation point will be lower. It gives however idea how more expensive is the KNX solution compared to the wireless home automation promotted by Google, Apple, z-wave or zigbee, especially for small systems. Note also that at this point we’ve build just the basic infrastructure for manual control, without web server and controller to manage the bulb remotly from internet or mobile device.

Installation diagram for KNX and the traditional poer wiring
Simplified diagram of the KNX and the power lines to light a bulb. The KNX power supply converts the 220V to 30V and through the green KNX cable supplies power to the KNX modules. The KNX cable is also used to communicate between modules. Which of them can receive and send information and interpret that information according to its task. In our example, when the KNX switch is pressed it sends a command to switch ‘ON’ on the KNX cable. All connected modules will receive the ‘ON’ command, but only the actuator module, which is programmed to execute the commands of this particular switch, will pay attention and execute it. Circuit breakers are always connected before KNX actuators. It is preferable to protect the KNX power supply separately.
KNX WAGO terminals
KNX terminals

The small black and red polarity terminals are specific to KNX products and are manufactured by WAGO / Jung. They are connected by the green KNX cable, and you are completely free to make the connection as you choose, as long as you keep the polarity – parallel, serial, in a star … It is important to never make a close loop with the KNX cable!

This freedom of connection allows you to route the cables as you wish in your home. You can even buy separate terminals to use as splitters where you need them.


ETS programming software

Programming is done with the only tool – the ETS software of KNX.org, whose legally purchased license costs around $1000! For small systems, a lite edition of ETS is available for $200. It allows you to create projects with up to 20 devices. There is also a free version that works with up to 5 KNX modules.

You should also add the price of an interface module between your computer and KNX (USB or Ethernet) – $150. It is used for programming and to connect your system to a PC monitoring interface or controller / web server for home automation.

KNX products and books on Amazon

KNX architecture

The KNX ecosystem provides all control functions for smart homes and buildings – lighting, heating and air conditioning, security, alarms, energy efficiency. Control can be local or remote – via the Internet, smartphone or tablet. The architecture of a functional KNX system includes:

  • Sensors (motion, presence, fire, CO, illumination, temperature, etc.);
  • KNX buttons, ultra terminals, binary inputs…;
  • Executive KNX actuators (relay outputs, contactors, dimmers, …);
  • System components (KNX power supplies, interface and gateway modules);
  • Software for monitoring and control and controllers – used for web servers, remote monitoring and control via applications or browser, set-up of scenes, events, notifications. The controllers also integrate Ethernet (CCTV, multimedia) and other protocols to create a single system.

KNX sensors and ultraterminals

Sensors

They provide input to the system (Inputs) about the status of physical values or the occurrence of an event. All functions are available.

Binary inputs
Discrete binary inputs, 8 channels. Turn signals from buttons, standard sensors or any other dry contact into KNX commands (telegrams).

KNX sensors
KNX switches, buttons and terminals

Buttons & control terminals

They also provide input to the system (Inputs) due to user interaction – pushing a button for lighting, dimming, adjusting the temperature of a thermostat, lowering the blinds, etc.
These are visible elements that are directly related to interior design. The choice is large – most ultraterminals from major manufacturers such as Legrand, Siemens, Schneider come with a KNX variant, and specialist brands such as Jung and Gira offer incredible switches and control panels.
Unlike standard installations, here the control buttons allow much more flexibility due to the simple wiring (a single KNX control cable). Instead of single or serial switches, real centralized control panels can be installed that can be configured to trigger the relevent devices in the home. For example, from one module with buttons in the living room, the user can switch On the room lighting, but also reduce or increase the temperature, switch on the lighting in the yard or unlock the outside door.

KNX actuators

They perform the physical action – switching contact, dimming, etc.

Switching actuators

The switching actuators have relay or transistor outputs for switching On and Off various loads – lighting, heating, etc. DIN rail mounted modules are between 2 and 20 channels. Each channel is a discrete output that is controlled individually via KNX communication messages (telegrams). The outputs have a switching capacity of up to 6A for the transistors and 10A or 16A for the relays. For switching higher power, the modules are used in conjunction with contactors. There are also specialized switch actuators for control of 3-phase loads, fan speed control. etc.

KNX switching actuators
KNX DALI Dimmers, heater controls, blinds and shutter controls

Other actuators

In addition to the discrete outputs and switch actuators, there are modules for dimming, blinds, shutters and rolling shutters contol, heating control mechanisms, alarms and signaling devices. There are also KNX interface modules for heating installations through which it is possible to operate gas boilers, heat pumps, etc.

Daikin KNX module
KNX module developped to control Daikin heat pumps.
Quality Mean Well power supplies for KNX

KNX System Components

Power supplies for KNX

They are produced for 160mA, 320mA and 640mA nominal current. In order to be certified, the maximum power consumption of each KNX module must be less than 10mA. Therefore, 160mA PSU can supply 16 modules, 320mA power – 32 modules and 640mA respectively 64 modules. If your budget allows, buy directly 640mA PSU – for a medium to large house you will reach 64 pretty quickly. If your system requires more than 64 modules, bus couplers (line connectors) are used.

Some KNX PSUs have additional white and yellow terminals. Those are for supply to KNX modules which, due to their functionality, have a higher power consumption than 10mA. Typical example are IP interface modules, which also have terminals for extra power. The KNX cable also has white and yellow wires, used only for secondary power supply for the time being, but also serve as a backup for future changes to the standard.

Some power supplies have 2 sets of red and black terminals and can supply two different lines. However, a bus coupler is still used between the lines.

KNX power supplies
KNX - bus couplers and IP couplers

Bus couplers

They connect two KNX lines. One line can have up to 64 points. When the installation needs more than 64 modules, it should be divided into separate lines. These lines are connected with bus coupler so that they can communicate with each other. In this way, an oreder (telegram) sent from a module in line 1 (powered by PSU 1) will reach the modules in line 2 (powered by PSU 2).

The bus couplers also reduce the data traffic throughout the system by filtering only the telegrams between the lines, using a filter table that is automatically created or manually set in the ETS programming software.

There is also a line connector over IP networks called the IP KNX Coupler or KNX IP Router. It serves as a communication between two remote buildings equiped with KNX systems, when the only possible physical connection is an Ethernet/Internet network. It creates an IP tunnel in which KNX messages flow from line in one building to line in the other.

Remote control & monitoring of your KNX system

Once your KNX system is installed and wired, it gives you the functionality of a traditional electrical system, plus much more flexibility to assign and distribute the control functions. You are of course interested in all fancy home automation features, like implementing fully automated scenarios or control and monitor your home from a mobile device. These are achieved with specialized controllers or PC and related software. In both cases, the functionality includes a web server with a graphical interface that allows remote management through Internet browser or client apps for iOS and Android.

Choosing the right control & monitoring platform is very important step. Here are 3 basic criteria to help you choose:

  • Functionality and scalability. Will you only manage KNX devices or will your smart home system include CCTV, intercom, SIP telephony, smart devices supporting other protocols (such as Zigbee and Z-wave wireless devices), multimedia management?
  • Openness and support. Do you want to fully know your system and develop it yourself, or is it going to be a “black box”, installed and maintained by a system integrator or other third party provider?
  • Price
Jung Smart Visu Server

Conclusion

KNX is certainly one of the most robust automation platforms for home and building automation. Its magic comes with a price. It is targeted to large new homes and buildings (hotels, hospitals), where requirments for reliability, security and size of the installation are a no go for cheaper wireless automation solutions.

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