The following files are available for download:
Schematics and PCB on EasyEda for the RS-bus feedback board (V4.1).
Schematics and PCB on EasyEda for the RS-bus feedback board (SMD version) (V4.2).
An Arduino RS-Bus Library (on GitHub)
For most people the EasyEda sources will be the place to go, since it allows the easy and direct ordering of (assembled) boards via jlcpcb. Total costs for 10 empty PCBs will be less than 5 euro, costs for 10 boards with assembled SMD components will be around 15 euro (excl. shipping). See also the page on PCB costs.
Software for the original C-code version can be dowloaded from here.
For historical reasons the original boards in Eagle (version 5.1) can be downloaded as well: