Hello Egor,
Probably there are no pull-ups inside the engine, but I think there is a design error
that outputs 8 bits on the databus when the mapper register is read. This while
the internal mapper register of the engine is probably 5 bits. So, when the mapper
register of the engine is read, it drives bit 7..5 high.
And yes, it is possible to make pull-ups inside a chip, I can't tell you now how this
is done, I have too look it up in some old school-books...
Now to the point with an external mapper, all the higher bits are read as one :
e.g.
Internal 512 Kbyte mapper
External 2048 Kbyte mapper
out &hfe,1
in &hfe -> &b11100001
This is with and without the external mapper!
I could be wrong, but I think with a 256 kbyte turbo-R ST even bit 4 will be high.
By adding a pull-up to a certain pin, the engine can be configured at power-up.
The engine wins the battle of the higher datalines because in series with the datalines
from the processor(s) to the cartridge connectors there are resistors.
(I think they are 100 ohm)
The engine doesn't have these resistors in it's way to the processor(s).
Greetings from Erik Maas
----------
From: Egor Voznessenski
Sent: Tuesday, May 26, 1998 1:44 PM
To: [EMAIL PROTECTED]
Subject: Re: Looking for Ram
Martial BENOIT wrote:
> At 00:27 26/05/98 +0200, you wrote:
>
> >The problem with the MSX turbo R is that the pull-up resistors in the
> engine (S1990) have a
> >too low resistance, which makes the current that high that even the
> low-bit produced by an
Pull-up inside the engine? Sounds VERY interesting.
> >external large mapper will be pulled up. In theory this can damage both
> the external mapper
> >and the S1990. I don't think that Panasonic will produce a new version of
> the S1990 to solve
> >that problem. I don't think that Panasonic will produce anything anymore
> for MSX. So we will
> >have to live with these problems and work around them with software
> routines! Whether we like
> >it or not :-(
>
> I use a self made upgrade using a 1MB SIMM 80ns (30 pins PC DRAM) and some
> 74HCxx chips to enable read and write to the 'missing' 3 upper bit of the
> mapper registers, and I haven't destroy my S1990 until now.
And this seems to prove my theory about S1990 actualy NOT outputting higher
mapperbits.
> It seem that you have the some data about the internal structure of the S1990.
>
> May I ask you if you could tell me what is the value of the pull up
> resistors for the 3 upper bits of the mapper registers?
>
Did anyone actualy TRY to read external mapper on the Turbo R?
application/ms-tnef
