Hi,
You have to explain what you mean with 'additional port'.
A real slot expander expands a primary slot to max 4 expanded slots. In
there memory can be switched to the z80 by the slot hardware.
This is memory space, not i/o space as is used for i/o ports. These are
limited to 256 in total whether a slot expander is used or not. All
cartridges in any slot and internal devices are always 'i/o port active' at
the same time.
An example of a fake (not real) slotexpander is the Elektuur design which is
nothing more than eight parallel wired slot connectors of which only one at
a time is made active with the slot select signal. Handy to avoid replacing
cartridges (and to avoid a cartridge from starting at boot time by making
that slot inactive, grab ROMS!). By the way: when building this Elektuur
device: add the busdir signal and remove the databus buffers.
Ofcourse it is possible to add a third slot connector by getting a connector
in parallel with one of the existing slot connectors and replace the slot
select signal with the appropiate slot select signal out of the computer,
primary (MSX1) or secundary (MSX2, where internal slots are expanded most
of the time.
In theory: You can have two slots completely wired in parallel, insert a
only ROM based cartridge in one and only i/o based in the other. But how are
you sure it is pure ROM or i/o? Not recommended, the chance of having
conflicts is big.
--
For info, see http://www.stack.nl/~wynke/MSX/listinfo.html
