Finally I have been able to prepare an adapter for the uart port at the rpi and check that rocrail works perfectly (so far). Hereafter a small video demonstrating the achievement.
Rocrail is using the ddx library to send the commands through the serial port of the rpi. The booster board (ord-3) is powering the test track. Peter Gilling, designer of the board, advised on reducing the value of the resistor R1 – I am currently using 390 ohms- , this is also convinent for usb-rs232 converters. He was also concerned about the rpi being able to generate the signal… This is something that needs further tests… so far, with just one loco and without launching rocview, it seems to work OK.
The raspberry has been adapted with a connector to generate the right voltage levels. It is based on this design I found on internet.
I am attaching hereafter the schematics and the board.
I have left some jumpers to select the signal being generated in the UART pins or other GPIO pins. In this way, if I want to play to generate the signal directly, and not though ddx, I could have a test board… this could also help to experiment and activate certain functionalities not available with usb converters -i.e. shortcut detection or programming track. In the same way, power can be selected (5v or 3v).
Note that the connections of this board would require a null-modem cable. The ORD-3 is expecting the signal in pin 3 (where the RS232 port of a PC would provide it), and the adaptor is generating the signal in pin 2. In fact, the adaptor is prepared to connect the rpi to a PC.
Obviously, it is possible to create a board with the right pin layout to directly interface the ord-3:
Last but not least the guideline to use rocrail in a raspberry pi has been updated: