How do I set a I/O breakpoint
Open the debugger window. In the bottom right is the stack and the breakpoint windows.
(see attached picture)
Right click to bring up the menu.
Add Breakpoint->IO Breakpoint...
You will see the io breakpoint window. (see attached picture)
Here I have chosen a default and it has filled in all the details for me.
You can enter your own details here, port, port mask, data, data mask to allow capturing of any i/o read/write from any port with any specific data.
The default will capture any write to 7fxx with data bits 7,6 set. The debugger will stop *after* the instruction, so in the dissassembly window move up to see it.
In the debugger window in the top right is the hardware details panel. Z80 should be visible by default. I/O port and I/O Data tell you what the z80 saw.