Perhaps
the following might help Gary…
In
my travels is working with the 80486-40 I noticed that with
one (of my two) SMB-V2’s it would not switch back to the Z80
using the Output a 1 to bit 0 of port 0EEH.
However
if I jumpered K3 2-3 so it used only 8 bit port addressing
and that immediately corrected the problem.
When
I set K3 to 1-2 (16 bit port addressing) the problem
returned. I suspected the problem is U31B is not getting
the address lines up in time on its pin 5 to clock the
correct data into U33. If I use a 74F138 for U27 and a
74F10 for U18 the problem disappeared. I.e. I could always
switch back to the Z80.
Could
you try these in your system.
John