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General information
Ever increasing demands made on the flexibility and convenience of electrical installations, combined with the demand to minimize energy requirements, have led to the development of building control. The bus technology used in these systems is based on manufacture-independent and internationally standardized technology: KNX. More than 100 manufacturers support this standard and have joined forces to form the KNX Association.
The member companies ensure the availability of bus-compatible products. This has made it possible for devices from various manufacturers to be used in a single KNX system.
Demand for more convenience and the fact that more and more is technically possible means that an increasing amount of time and effort is being devoted to electrical installations. While conventional electrical installation technology has reached the limits of its capabilities. GAMMA instabus, the intelligent building control from Siemens based on KNX has made it possible to satisfy these comprehensive demands with solutions that are both easy to manage and affordable.
System arguments
In conventional electrical installations, each function needs its own cable and each control system a separate network. By contrast, GAMMA instabus allows all operational functions and processes to be controlled, monitored and signaled over a single common cable. This means that the energy feeder can be routed directly to consumers without any detours.
Not only does this reduce the amount cables required, it also has other huge advantages: electrical installations in buildings are far simpler to install and it is also easy to add any subsequent extensions and make modifications. If the purpose or configuration of a building is changed, the GAMMA instabus system is easy to adapt by simply reassigning the various bus devices (changing their parameters), without the need to lay any new cables. These parameters can be reassigned using a PC connected to GAMMA instabus and the configuration and commissioning software ETS (Engineering Tool Software).
With the right interfaces, GAMMA instabus can also be connected to the control centers of other building automation systems (e.g. SICLIMAT X) or to a public telephone network (e.g. ISDN) or using a LAN/Internet connection. It is therefore just as cost-effective to use the GAMMA instabus in the family home as in hotels, schools, banks, office buildings or complex non-residential buildings.
Transmission technology
GAMMA instabus based on KNX is a distributed, event-controlled bus system with serial data transmission for the controlling, monitoring and signaling of operational functions.
All the connected bus devices can exchange data over a common transmission path, the bus. Data is transmitted in serial mode and in compliance with precisely defined rules (the bus protocol). The data to be transmitted is packed into a telegram and sent over the bus from a sensor (the command output) to one or more actuators (the command receiver).
Each recipient acknowledges receipt of the telegram when the transmission is successful. If no acknowledgement is issued, transmission is repeated up to three times. If the telegram is still not acknowledged, the send operation is aborted and the error noted in the memory of the transmitter.
Transmission of data using KNX is not electrically isolated as the power supply for the bus devices (24 V DC) is transmitted at the same time. The telegrams are modulated on this direct voltage, whereby a logic zero is transmitted as a pulse. The omission of a pulse is interpreted as a logic one.
The individual data of the telegrams are transmitted in asynchronous mode. However, transmission is synchronized by start and stop bits.
Access to the bus as the shared physical medium of communication for asynchronous transmission must be controlled unambiguously. In the case of KNX, the CSMA/CA procedure is used for this purpose. The CSMA/CA procedure guarantees collision-free access to the bus without any reduction of bus data throughput.
All stations listen in but only those actuators actually addressed respond. If a station wants to transmit, it first has to listen in and wait until no other station is transmitting (Carrier Sense). When the bus is unoccupied, any station can begin a transmission operation (Multiple Access).
If two stations begin to transmit simultaneously, the higher-priority instantly asserts itself on the bus (Collision Avoidance), while the other station pulls back and restarts the transmission operation some time later.
If the two stations have an identical level of priority, the one with the smaller physical address asserts itself.
Addressing
Every letter needs an address in order for it to be correctly delivered by the postal service. The addressing of bus devices is similar, but the form used for postal purposes is unsuitable in this case.
During configuration with the ETS, each bus device is assigned its own physical address with which it can be uniquely identified; just like a postal address is a unique ID for the recipient of a letter. However, the physical address has to be expressed in the language of the bus and is based on the topological structure of the KNX system.
Physical addressing is used by the ETS only for commissioning the individual bus devices or for servicing and diagnostics activities. In this case the addressing is performed along the same lines as for the postal delivery service.
By contrast, the KNX system uses a different address for telegram traffic: the logical or so-called group address. This address is not based on the bus topology but on the operational functions (applications) of the building.
Unlike the postal service, which delivers a letter to the recipient's address, the configured group address is written into each telegram sent by the transmitter. Every bus device listens to this telegram, reads the group address contained in it, and checks whether the telegram is addressed to it or not.
The group address to which a bus device should respond is assigned during configuration of the KNX system using ETS. Unlike the postal delivery service, several group addresses can be assigned to one bus device.
When a bus device is listening to a telegram on the bus, it will always receive the telegram if it responds to the group address entered in the telegram. If not, it will discard the telegram as not being intended for it.
Topology
Up to 64 bus-compatible devices (stations) can be connected to and operated on the smallest unit of the KNX system, i.e. on a single line. Using line couplers connected to the so-called main line it is possible to bundle up to 15 lines in an area.
Fifteen areas can be joined together by means of backbone couplers, which are connected to the so-called backbone lines, in order to form a larger unit.
Interfaces (gateways) to third-party systems (SICLIMAT X, LAN, etc.), or additional KNX systems are connected to the backbone line.
Although more than 14,000 devices can be interconnected in a single unit, the clear-cut logic of the system is preserved. Telegrams only ever overstep the interfaces to other lines and function areas if they are needed in those areas. This minimizes the telegram load on the main line. Line/backbone couplers carry out the necessary filter function.
The physical address is based on this topological structure: every device can be uniquely identified through the specification of its area, line and device number. For assignment of the devices to the operational functions the group addresses are divided into main groups and subgroups.
During configuration it is possible to divide the group addresses for different management functions into as many as 14 main groups, e.g. for
- Lighting control
- Shutter/blind control
- Room control for heating, ventilation, air conditioning.
Each main group can include as many as 2048 subgroups, to suit the user's requirements. This means that each device is able to communicate with every other one.
Technology
Each line requires its own power supply unit for the devices and is therefore self-sufficient.
The Siemens power supply unit supplies the individual devices on the line with SELV (safety extra-low voltage) of 24 V DC and, depending on the version, can be loaded with 160 mA, 320 mA or 640 mA. It features both voltage and current limiting and is therefore short-circuit resistant. Short system interruptions are jumpered with a buffer period of 200 ms.
The bus load depends on the type of devices connected. The devices are ready for operation at a minimum of 21 V DC and typically draw 150 mW from the bus. If there is a concentration of a large number of bus devices in a single location, the power supply unit must be located in the near vicinity.
A maximum of two power supply units are permissible in one line. A minimum distance of 200 m of cable length must be observed between the two power supply units.
The length of a cable plus all junctions must not exceed 1000 m. The distance between a power supply unit and a device must not exceed 350 m. In order to guarantee no telegram collisions, the distance between two devices should be limited to a maximum 700 m.
The bus cable can be laid parallel to the mains cable. It can be looped and branched. A cable terminating resistor is not required. The devices are connected to the bus by means of either pressure contact or bus terminals. Connection by means of pressure contact is achieved by simply snapping the devices (designed for installation in distribution boards) on to the TH 35 EN 60715 standard mounting rail with integrated data rail. Transition from the data rail to the bus cable is effected by a connector. The bus cable is connected to surface-mounting, flush-mounting, wall-mounting, ceiling-mounting and built-in devices by plugging on the bus terminal.
Devices
A station in flush-mounting design is typically made up of a universal bus coupling unit (BCU) and a task-specific bus terminal device (BE, such as a pushbutton or display) that exchanges information with the BCU over the user interface (UI). The BCU receives telegrams from the bus, decodes them and actuates the BT. Conversely, the BT sends information to the BCU, which encodes it and sends it as a telegram onto the bus.
During configuration and commissioning with the ETS the BCU receives the parameterization data for the function that is to be performed. For this purpose the BCU is equipped with a microprocessor (MP) with a non-volatile ROM chip (Read Only Memory), a volatile RAM chip (Random Access Memory) and a non-volatile EEPROM chip (Electrically Erasable Programmable ROM) that cannot be overwritten.
The ROM contains the system-specific software that cannot be changed by the user. The parameterization data for the function of the BCU to be performed are saved by the ETS in the EEPROM. The current data are saved by the MP in the RAM.
The assignment of the UI pins differs on the various BTs. This ensures that a BT connected through the UI is able to communicate error-free with the BCU when the relevant application program has been loaded by ETS in the EEPROM of the BCU.
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System data |
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Bus cable |
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mm2 |
YCYM 2 x 2 x 0.8 |
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Cable length |
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m |
max. 1 000 (including all junctions) |
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m |
Max. 700 |
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m |
Max. 350 |
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Side-by-side mounting necessary |
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Bus devices |
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Max. 15 |
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Max. 15 |
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Max. 64 |
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Topology |
Line, star or tree structure |
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Power supply |
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V DC |
24 (SELV safety extra-low voltage) |
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One power supply unit (160, 320 or 640 mA) |
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max. two power supply units at a distance of at least 200 m |
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Transmission |
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Distributed, event-controlled, serial, symmetric |
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bit/s |
9600 |
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Device features(unless otherwise specified) |
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Degree of protection according to EN 60529 |
IP20 |
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Protective measure |
Bus: safety extra-low voltage SELV 24 V DC |
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Overvoltage category |
III |
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Rated insulation voltageUi |
V |
250 |
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Degree of pollution |
2 |
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EMC requirements |
complies with EN 50081-1 and prEN 50082-2 (severity 3), |
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Resistance to climate |
prEN 50090-2-2, KNX/EIB manual |
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Operating conditions |
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For fixed installation indoors, for dry rooms and |
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°C |
-5 to +45 |
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% |
Max. 93 |
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°C |
-40 to +55 |
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% |
Max. 93 |
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Approvals |
KNX/EIB certified |
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CE marking |
Compliant with EMC Directive (residential and non-residential buildings), Low Voltage Directive |
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