Central bus power supply - Basic Documentation - RMK770-1

Communication via the KNX bus

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Air conditioning controller
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When one or more power supply units (PSU) are used for the bus power supply, this is referred to as a "central bus power supply". This type of bus power supply is required in the following circumstances:

  • The max. allowed 8 Synco devices with DPSU (Decentral bus power supply = On") is insufficient to cover the power consumption of the devices without a bus power supply.

  • Synco devices and third-party devices are installed on the same line.

  • Only devices without a bus are installed on the same line.


On a line with a central bus power supply, the following devices must not be used to supply power to the bus:

  • RM… controllers

  • OZW775 central communication unit

  • QAX9x3 central apartment unit

These Synco devices must be set to "Decentral bus power supply = Off" to prevent them from interfering with bus communications.

Power supply unit PSU

At present, power supply units for 160, 320 und 640 mA are available. The power requirements of the devices must be calculated as a means of determining the required power supply unit, PSU See the relevant values on the previous page.

For a line with 64 bus devices, each with a power consumption of 5 mA (64 x 5 mA = 320 mA), a PSU of 320 mA is sufficient.

Siemens power supply units

For KNX networks, the following PSUs by Siemens are recommended.

Order types:

  • 5WG1 125-1AB02, short code N125/02: 160 mA (with integrated choke)

  • 5WG1 125-1AB12, short code N125/12: 320 mA (with integrated choke)

Product data:

  • Operating voltage: AC 120…230 V, 50…60 Hz

  • Bus voltage output: DC 29 V (21…30 V, with choke)

For KNX networks with IP routers, the following Siemens power supply unit is recommended:

Order type:

  • 5WG1 125-1AB22, short code N125/22: 640 mA

Product data:

  • Operating voltage: AC 120…230 V, 50…60 Hz

  • Bus voltage output: DC 29 V (21…30 V, with choke)

  • Auxiliary voltage output for IP router: DC 24 V (12…30 V, without choke)

For further information see the product description and description of functions (enclosed with the device) or under: http://www.siemens.com/knx-td


A DC 29 V voltage with choke is required for the bus power supply. The DC 24 V auxiliary voltage for the IP router requires a voltage source without choke.