Posted By
TLC
on 2022-11-04
06:18:07
|  Re: Let's remake the DigiBlaster!
Good, at least that part of the mystery seems to be solved  .
Emulation - well, from software, the DigiBlaster merely looks like a pair of registers, i.e. a 8 bit D/A register at base +$1E, and another 8-bit A/D register at base + $1F, respectively. (By base + $1E I mean both $FD5E and $FE9E etc., as usual in this card). IMHO it's quite trivial to emulate.
Address decoding - I think the decoding logic has to provide at least three CS' signals, i.e. +$1E_write, +$1F_read, and also +$1F_write. The reason for +$1F_write is that 1.) the digitizer code in the Digital Composing Kit very obviously writes the $FD5F address every once after it had read it, which aligns well with the fact that 2.) successive approximation A/D converters typically need a strobe to actually start the conversion process.
(Note also - the address decoder needs to exclude the DigiBlaster's assigned 2 addresses from the onboard SID's chip select.)
BTW, if I'd have to make a guess about the A/D, I'd probably say the ZN427. (Now thinking it over, that's gonna be a somewhat challenging quest... I mean not just technically but logistically... AFAIR poor Ferranti i.e. the producer of the ZN series has been out of business for the last 30 or so years now... forget it; did a bit of search, and, fortunately, Ferranti's semiconductor business appears to have survived (--> it became Zetex) ).
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