Re: Looking for YMF262

From: Ruud Baltissen (
Date: 1999-07-05 22:58:27

Hallo allemaal,

> On Fri, 2 Jul 1999, Nicolas Welte wrote:
> > > Hmm. now that I have worked out the bizarre implementation of the FM sound
> > > chip on the 65xx bus, I doubt very much that a Soundblaster card could be
> > > grafted onto the Commodore bus without at least some modifications on the
> > > Soundblaster pcb.
> > 
> > Why? The ISA bus should also have the separated READ and WRITE lines,
> > and all the address lines are there. The interface to the SB card could
> > be similar to the C64-OPL2 interface, with an added address encoder for
> > the SB card. Instead of just decoding the C64 addresses to the OPL2 chip
> > select you would have to create an ISA bus I/O address for it. Somewhat
> > dependent of the R/W signals of course, because of that strange mapping
> > on the Sound Expander. Doing it with discrete TTL components might
> > actually be hard, but it should be possible to use some programmable
> > logic like GALs. Not that I had any experience with them :-(
> Well I must admit I didn't know ISA had seperate read and write lines. In
> that case, the decoding of the YM3812 on a Soundblaster is probably simple
> enough not to get in the way of the 65xx bus. I'll buy a few ISA
> connectors today ;) Basically, I/O2# has to be 'un-decoded' to appear as
> 22xh on the ISA address bus. Assuming READ# and WRITE# are connected
> straight through to the FM chip, they can be decoded in the usual SFX way
> on the SB->C64 bridge card.

I once connected a MGP-card to my C64. The card disappeared during moving from 
one houde to another. 
If the sound blaster only uses I/O, one half of a 74LS39 will do the trick. 
Connect the PHI2-clock with input A and the R/W-line with input B. Output C 
reflects the needed WR-line, output D the needed RD-line. Connect the IRQ-input 
thru an invertor with IRQ5 of the card. The only thing I don't have a solution 
for is the DMA. 
You must hardwire the addresslines A8 and A9 so that the card sees the range 
$200-2FF. The AEN-line is connected to PHI2-clock AFTER it has been inverted.

For those interested how I connected the MGP-card which needs RAM-addressing as 
well: I added a 8255. Two ports I used for A8..A19 and one line I used for 
telling it was an I/O- or Memory-operation. The other lines I used to read the 
IRQ-lines of the ISA-slot. These lines were connected to the IRQ-input of the 
C64 thru a NOR-gate.

Groetjes, Ruud

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