A Customizable TP-UART Interface

The goal of this project was to design a network interface for the KNX/EIB twisted pair medium (KNX TP1/EIB TP), with a focus on - but not limited to - communication-related R&D tasks. For evaluating protocol extensions and performing communication diagnostics, the ability to go beyond existing protocol specifications is as important as being capable of acting in precise conformance.

The TP-UART IC is a very convenient basis for such a project. This smart transceiver handles considerable parts of the medium access control layer. As a consequence, the host is relieved from most tight timing issues. Still, it remains in full control of all higher level protocol aspects. In this respect, the TP-UART leaves much more leeway than standard EIB bus couplers. However, the elements of the TP-UART host protocol that are still time-critical are hard to handle if the host is a PC. Also, the TP-UART datasheet leaves open some questions concerning this protocol.

Therefore, the decision was made to enhance the interface with a microcontroller that could eliminate - or at least check - the timing constraints of the TP-UART host protocol. Such a design would also enable communication testing on an even lower level by making use of the TP-UART "analog mode", where only the transceiver part is used and the host takes over the entire data link layer. When creating the design, care was taken that the hardware would also support other uses. Signal routing can be selected via jumpers and some LEDs, switches and buttons are available for user interaction. The board can be populated fully or with only a subset of components depending on the task; also, it can be used both with and without an enclosure. The microcontroller can be programmed in-system. Since the design is also released as fully "open hardware" (and can serve as a tested template for own designs), we think it is truly "customizable" as promised in the project title.

The platform was successfully put to use to solve the original problem. Tests were made to provide additional information on the host protocol, a lightweight protocol extension was designed, and this extension was implemented.


Details and Download

All key results and concepts are discussed in the paper "Design of an enhanced TP-UART based KNX PC interface" by Georg Neugschwandtner and Andreas Fernbach. It was presented at the KNX Scientific Conference in St. Katelijne-Waver in 2008.

Detailed information on the hardware and related downloads can be found on the ACTIface project site on SourceForge .

The authors can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it. and This email address is being protected from spambots. You need JavaScript enabled to view it. .

ACTIface in a custom enclosure

ACTIface block diagram
ACTIface 1.1 PCB
Protocol sequence diagram


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