David Heremans wrote:
>
> > Some time ago I noticed that Turbo-R machines does that with all MSX
> > system
> > reserved I/O port ... Then if you try to read any I/O port that has a
> > internal
> > pehiperal installed the S1990 chip engine will block output from the
> > cartridge.
> > I guess it's made this way to avoid "totem pole" (bit conflicts) of
> > S1990
> > internal buffer with the internal machine devices such as PPI, VDP,
> > PSG , Mapper, etc ....
> >
>
> This seems verry unlogical, what you say would imply that you can't read
> your vram from port #98 for example.
> Or do you mean that it has the resistors on all external slots?
> Can somebody confirm your story or can you give example where it blocks
> those bits?
No , It's not unlogical, because V9958 is an internal pehiperal and
is controlled by S1990. If you plug another VDP on the same port using
the external slot you will be able to write any data to it, but if you
try read the vram you will get the vram contents of the internal V9958.
Simple , don't it? (Yeah ... I really tested it and it went as I said.)
I guess read to external mapper registers are blocked by the same
reason
that other ports are blocked. If you look at T-R schematics you will
notice that S1990 does all buffering for the slots and also
controls the mapper register. That should de the only explanation
for the hardwired prohibition to read the external slots on the ports
that already have devices latched internally...
Cya MSXers ...
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