Firstly, if you don’t know or understand what this mod is for, have a quick read of this article 🙂
As mentioned in that article, the floating bus behaviour is not present on +2A or +3 machines as the split bus architecture was implemented internally in the gate array using proper tri-state buffers instead of 8 x 470 ohm resistors like earlier models. This prevents software that relies on floating bus behaviour from working properly.
There is a very simple mod to reinstate partial floating bus behaviour however – and it’s beautifully simple.
All you need is a single 470 ohm resistor and a soldering iron. To perform the mod, simply connect one end of the resistor to pin 17 of IC4, and the other end to pin 17 of IC6.
This has the effect of bridging a single bit (D7) of the data bus in exactly the same manner as it is on earlier models. That single bit change allows software that attempts to read the floating bus to detect when video reads as occurring.
It’s not a full floating bus implementation however, as the value read will only change between 0xFF and 0x7F. It’s good enough to fool most software that relies on the effect though!