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I will put pads in for the capacitors - I remembered you and Tim trying them but couldn't recall if they ended up being needed.
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As the firmware currently stands, yes, they really are needed and I am not personally convinced that that problem has a firmware solution since there is no way for the firmware to influence that problem - as the PIC ports used don't have a genuine hands-off (tristate) mode, and can only be input or output. That's why I suggest adopting the same solution as the original VDU - 'proper' tristate buffers in between the VDU 'address bus' and the actual address bus.
It might still be possible to play with the timing of the point at which the port pins switch from input to output, and, as has been pointed out before, the port pin directions can only be changed one port at a time, so maybe changing the order in which the A0-A7 port and A8-A11 port have their directions changed over could also make a difference.