• The KNX PL-Link bus must be inside the building.
  • The KNX PL-Link bus allows communication from PXC4, PXC5, and PXC7 automation stations to a maximum of 64 devices on the KNX bus.
  • The number of devices is limited by the number of BACnet objects on the automation station, see System functions and configurations and the available bus power. Bus power is incremented during engineering with the ABT Site tool.
  • The KNX PL-Link bus basic version comprises one cable and two stranded bus wires.
  • The PXC4, PXC5, and PXC7 automation station has one internal bus power supply of 50 mA.
  • The KNX PL-Link is physically based on the KNX bus (Konnex) .
  • In KNX PL-Link networks area/line couplers and IP routers are not allowed.
  • Interconnection of automation stations via KNX PL-Link is not admissible. The connection is done exclusively via Ethernet switches or MS/TP.
  • The polarity of the KNX PL-Link bus conductors must be correct (KNX terminals + and –).
  • The PXC4, PXC5, and PXC7 automation stations do not support third party devices integrated via S-Mode (ETS engineering).

Bus power supply

A bus power supply is required for KNX bus communications. Throttled voltage DC 29 V is used.

Internal KNX PL-Link power supply

The automation stations have an internal bus power supply. It is switched on by default.
PXC4, PXC5, PXC7: 50 mA to supply approximately 5 KNX PL-Link devices

Important note: If an external supply is used, the internal supply must be switched off via ABT Site tool.

External bus power supply

If the power supply of 50 mA is insufficient to cover the power demand of the connected devices, the internal bus power supply must be switched off via ABT Site tool and replaced by an external bus power supply unit (PSU).

Power supply units for 80,160, 320, and 640 mA are available in specialty stores. The total power supply for the devices must be calculated to determine the appropriate power supply size. Comply with the corresponding details in the datasheet.

A 640 mA power supply unit is sufficient to supply 64 devices on the KNX bus with an average power demand of 10 mA each.

Parallel operation:

  • Parallel operation of external bus supplies with the internal bus supply of an automation station is NOT allowed.
  • In theory, parallel operation of external bus supplies is possible. However, check if the specific PSU is allowed to be operated in parallel with other PSUs.
  • Depending on the type, a minimum cable distance is required between two PSUs.

Siemens power supply units:

We recommend the following Siemens power supply units for KNX networks:




5WG1 125-4AB23, short designation RL125/23

80 mA

With integrated throttle.
Parallel operation with other PSUs is allowed.

5WG1 125-4CB23, short designation JB 125C23

80 mA

With integrated throttle.
Parallel operation with other PSUs is allowed.

5WG1 125-1AB02, short designation N125/02

160 mA

With integrated throttle.
Parallel operation with other PSUs is allowed.

5WG1 125-1AB12, short designation N125/12

320 mA

With integrated throttle.
Parallel operation with other PSUs is allowed.

5WG1 125-1AB22, short designation N125/22

640 mA

With integrated throttle.
Parallel operation with other PSUs is allowed.


  • Operating voltage AC 120…230 V, 50…60 Hz.
  • Bus supply output DC 29 V (21…30 V, throttled)

Bus topologies

64 devices on one line:

Up to 64 devices with KNX PL-Link can be installed on one line (main line as well). No restrictions apply to the type mix.

  • There is no need to calculate the bus load number E for up to 64 devices.
  • The maximum of 64 devices is also valid when devices requiring less power are used.

Permitted bus topologies:

Permitted bus topologies are: Tree, line, and star topologies. These topologies can be mixed as needed. However, ring topologies are not allowed.

Tree topology:

The tree topology is preferred if a large network must be created.

Branching and connection variants:

N1 .. N8 Devices:


Device with screw terminals


T branch with bus terminals


Device with spring cage terminals


Bus lines:

The bus lines (= twisted pair) are connected via PL+ (red) and PL– (black).

Bus cable selection:

Choose the bus cable as per country-specific offerings. Comply with the indicated values in section "Technical data KNX PL-Link".

AC 24 V for field supply can be provided in the same (2 x 2 stands) or in a separate cable.

Recommended bus cables are available on the KNX homepage:


Recommended cable types:

  • 1 x 2 x 0.8 mm (e.g. Belden YE00819 or YE00905 - non-halogen).
  • 2 x 2 x 0.8 mm (e.g. Belden YE00820 or YE00906 - non-halogen).

Bus cable screening:

Bus cables without screen are permitted. The screens available for bus cables do not need to be connected.

If interference is expected on the KNX bus, use a cable with screen. Connect the screen as per standard installation rules.

Bus cables specified by KNX:

The indications for distances and line lengths in a network are designed for bus cables specified by KNX.

Admissible cable lengths:

Comply with the following distances:

Network with internal power supply from the primary stations



Distance between device and internal supply

Max. 80 m (260 ft)

Distance between devices

Max. 80 m (260 ft)

Total length of all lines on one line

Max. 80 m (260 ft)

Network with external power supply (PSU)

Distance PSU to PXC with switched off internal supply

Min. 0 m

Distance device to next PSU

Max. 350 m (1150 ft)

Distance between two PSU operated in parallel

Min. 0 m for the Siemens power supply modules recommended in section "Bus power supply"

Min. 200 m (650 ft)

Distance between devices

Max. 700 m (2300 ft)

Total length of all lines on one line

Max.1000 m (3280 ft)


Important: The bus conductors must NOT be inverted (KNX terminals + and –).

  • At least one supply (internal or external) is required for each line and max. two supplies (external) are allowed per line.
  • Install the power supply unit as close to the network center as possible to achieve maximum line size.
  • The distance between the device and the next neighboring PSU may not exceed 350 m (1100 ft).
    Even if the power demand from the devices does not require it, two power units must be used depending on the line length.


Wiring KNX bus polarity:

Check the bus wiring prior to commissioning and make sure that the bus line polarity is not interchanged (KNX terminals + and –).

Operating voltage:

Check the operating voltage wiring to make sure that the devices are connected to AC 24 V or AC 230 V (as per technical device information). Apply operating voltage only after this check.

Bus power supply:

After switching on operating voltage, check whether bus power from the automation station or the PSU is available.

Technical data KNX PL-Link

KNX bus:

Transmission medium (bus cable)

TP (twisted pair)

Baud rate

9.6 kbps (fixed for TP)

Bus line polarity

PL+, PL– (not interchangeable)

Bus terminating resistor

Not required

Communication signal:

The communication signal is transferred symmetrically, i.e., as a voltage difference between the two bus lines (and not as a voltage difference to the earthing potential). The sign preceding the voltage between PL+ and PL– determines signal values 0 and 1.

KNX bus cable:

Cable type

2-wire, stranded (one wire pair)

or 2x2-wire, stranded

or spiral quad

Wire diameter

Min. 0.8 mm (AWG20)

Max. 1.0 mm (AWG18)

Line resistance

20 … 75 Ω/km

Specific capacity

10 …100 nF/km at 10 kHz

Specific inductivity

450 …850 µH/km at 10 kHz


Not required

PXC4, PXC5, and PXC7 devices do not have a connection for bus cable screens.