Distributed bus power - Basic Documentation - RMK770-1

Communication via the KNX bus

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Air conditioning controller
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Basic Documentation
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A bus power supply is always required for bus communications. There are two options:

  • Distributed bus power: DPSU = Decentral Power Supply Unit

  • Central bus power supply: PSU = Power Supply Unit


In any one line (including a main line) either the DPSU or PSU may be used.

Devices with a bus power supply

The following devices are delivered factory-set with the bus power supply enabled, and supply power to the bus.

  • Synco 700 RM…controllers DC 29 V, 25 mA

  • QAX9x3 central apartment unit: See the caution notice below .


Since the devices which are factory-set to "Decentral bus power supply = On" are distributed on the bus, they are referred to as "Distributed bus power supply units" (DPSU).

Devices with the setting "Decentral bus power supply = On" must not be installed on the same line as third-party devices. In such cases, the devices must be set to "Decentral bus supply = Off" and a power supply unit must be installed (see next section "Central bus power supply").

In devices with a bus power supply, the bus power section is electrically isolated from the KNX bus.


In addition to its own intrinsic consumption, the QAX9x3 central apartment unit when set to "Bus power = On" delivers the bus power supply for one OZW771 central communication unit or OZW772 web server, or for the OCI700 service interface.

If other devices are installed on the same line, the central apartment unit must be set to "Bus power supply = Off". In this case, the bus power must be supplied by setting other devices such as the RM… controllers to "Decentral bus power supply = Off", or by installing a power supply unit.

Devices without a bus power supply

Devices without a bus power supply

  • QAWZ40 room unit

  • RXB/RXL room controllers

  • Room thermostats RDG/RDF/RDU

  • RMZ792 bus operator unit

  • OZW771.xx central communication units

  • OZW772.xx web server

  • OCI700 service interface

  • Area/line couplers and IP router


Devices without a bus power supply require devices on the same line with a DPSU or a power supply unit, PSU.

Rule of thumb

The rule is: One device with a DPSU can operate two devices without a bus power supply plus the OCI700 service interface plus the OZW771 central communication unit.

Per line, the rule is:

1 device with a DPSU + 2 devices with no bus power supply + OCI700 + OZW771

up to a maximum of

8 devices with a DPSU + 16 devices with no bus power + OCI700 + OZW771.


An RM… controller with a DPSU delivers a net current of 25 mA. This covers the power consumption of 5 devices without a bus power supply (5mA each required).

8 devices with a DPSU on one line deliver 200mA. This covers the power consumption for 40 devices or 38 devices plus OCI700 plus U/W771.


Even with devices having a low current requirement (< 3 mA), the number of devices in a line must not exceed 64.

See the data sheets for the power consumption of the new Synco devices.