I now have a working T300 with the rim being controlled by an Arduino Nano and it has solved my random button activation problem.
I got pretty much all the technical information from this guy who has done a fantastic job to help people create a custom rim.
http://rr-m.org/blog/hacking-a-thrustmaster-tx-rw-gaming-wheel-with-arduino-uno-part-1/
I had previously tried using an Arduino Nano emulating the T300 rim shift registers but I had problems with random gear shifts which I recently found out was caused by a faulty cable that I had installed. I replaced the cable and have been testing for about 4 weeks and I have not had 1 single random button press.
When putting it together and testing I found that if I wiggled the connection of the SS (slave select) input to simulate a bad connection then it would randomly activate “all buttons together” including the R3 and L3 which are on the base and that is what confused me earlier and made me think it could also be a problem in the base somewhere.
The shift registers are binary 1 for off and depressing a button will ground the input forcing it to a binary 0 which is on. The button presses are stored in 3 bytes and the info is transmitted 1 byte at a time (that is my understanding). The lsb of each byte in default button map is
Byte 1 lsb - Gear shift up
Byte 2 lsb - Menu
Byte 3 lsb - Dpad-up (brake bias on PS4)
So if the SS has a bad connection then all the bits go to zero then the PS4 or PC will read the lsb first and, according to what I have read, the above buttons are the ones people have the most problems with.
So the random button activation may be caused by either a bad connection or the chip used for the slave shift registers having communication failures with the master on the base. In my case it wasn’t the din connector as I previously took that out and soldered the wires through so maybe it was a bad connection somewhere on the rim or a faulty shift register.
PS4 users will have more random button presses than PC users as the PC does not read the d-pad inputs and also there is no menu button for the PC.
PS4 users can map another function to the d-pad up like a horn or lights if the brake bias random activation affects them. I don’t think it is possible to map the menu button though.
Perhaps not an easy solution for most people as building the wheel rim is not a simple task.