A customizable TP-UART interface
The RS-232 part is powered by the DTR and RTS lines (the PC must set DTR and clear RTS). For those cases where this is not possible, the package includes an optional power supply board.
The microcontroller can be left out, with the TP-UART host controller interface being directly connected to the PC. In this configuration, ACTIface is a "drop-in" replacement for the simple TP-UART interface board.
On the other hand, the board could also be assembled without the RS-232 interface to be used as a standalone KNX TP1 node (with the proper firmware, not provided).
The Siemens TP-UART on ACTIface handles the basic frame format and most of the low-level timing. Its host-side protocol is described in its datasheet (as of 06/2010, the latest version is dated 20.10.05 and available here). You will need some information from the KNX standard to make sense of the actual frame contents or to generate your own (meaningful) frames for transmission. The KNX standard itself is not available as a free download, but there are several books and open-source software projects where you can find this information.
The TP-UART host protocol is the protocol "spoken" by ACTIface if no microcontroller is fitted. Since the TP-UART datasheet leaves open some questions regarding this protocol, we made quite some tests and researched a couple of additional details. We have documented them in our paper.
If ACTIface is fitted with a microcontroller, the microcontroller firmware determines how ACTIface talks to the host connected at the RS-232 interface. The firmware included with the download package (which is also described in this paper) leaves the TP-UART host protocol almost untouched - with the exception of adding an end-of-frame indicator. In the original TP-UART host protocol, this information is only available to the host by monitoring a timeout in the millisecond range.