KNX TP

A certified KNX transceiver is required for connection to KNX TP. Despite the fact that KNX transceivers usually have a UART interface, a direct connection to a Linux CPU is problematic. For selective L2-ACK, a device must already decide during the reception of a telegram whether it is to be acknowledged on the bus. This information must be transmitted to the transceiver in good time via the TX path of the UART interface. In complete devices, serial ports are rarely led out.

In addition, galvanic isolation from the KNX bus is usually required.

To fulfill these requirements, kTux TP uses a USB interface. The USB interface is available in the form of a USB stick (KNX USB Interface 332) or as a module (KNX USB Module 322) for post connectors. However, rail-mounted devices (KNX USB Interface 312) can also be used. Interfaces from other manufacturers are not compatible. The USB chip can also be supplied as a pre-programmed controller on request.

KNX Secure requires an extended interface for persistent storage of the Secure Counter, which is also available as a stick or module (type stick 333, module 323).

KNX RF

The KNX radio protocol can be implemented with various standard radio transceivers for the ISM band at 868.3 MHz. Most transceivers use an SPI interface for the transmission of telegram data in NRZ mode (Non-Return-to-Zero). This means that the transceiver permanently transmits the received bits without interpretation. The start and end of a telegram is also not detected in NRZ mode.

The entire evaluation first takes place in the microcontroller, which permanently samples the data stream in order to recognise a possible telegram start. Once this has been done, the rest of the data is decoded to Manchester and the checksums of the individual blocks are verified. Only when a telegram has been received completely and without errors can the data link layer provide the telegram in cEMI (common External Message Interface, see KNX Standard Vol. 3, EMI-IMI) format and forward it to the higher levels in the KNX stack. Telegrams are sent analogously in the opposite direction.

The permanent sampling of an SPI data stream would place an enormous load on a Linux CPU and would only be possible in kernel mode anyway. For this reason, kTux RF, like kTux TP, uses a USB interface. This is available in the form of a USB stick (KNX RF USB Interface 340).

KNX IP

The connection to Ethernet is already integrated in almost all Linux platforms. A typical Linux board has a LAN interface with an RJ-45 socket. The necessary drivers are also implemented in Linux and offer an API as IP sockets.

The kTux IP is based on these IP sockets and does not require any additional hardware. For KNX Secure, a USB module is required for the persistent storage of the Secure Counter.