Re: Ultima 3 "Mockingboard" version.

[Linards Ticmanis <ticmanis@coli.uni-sb.de>]

"Michael J. Mahon" wrote:
> Actually, it's simplicity itself.  Each PSG is controlled by a parallel
> port of an associated 6522 VIA.  As Linards notes in a later post,
> the PSG is pretty easy to control, and the Mockingboard does nothing
> but interface it to the Device Select addresses on the Apple.

So basically all there is to it is:

Low address lines ==> VIA register select lines.
Device select AND NOT A7 ==> VIA 1 Chip select.
Device select AND A7 ==> VIA 2 Chip select. (this one requires one NOT
gate.)
VIA 1 Port A ==> PSG 1 Data/Address lines
VIA 1 Port B, lower 3 bits ==> PSG 2 Control Bus lines. (incl. Reset?)
VIA 2 Port A ==> PSG 2 Data/Address lines
VIA 2 Port B, lower 3 bits ==> PSG 2 Control Bus lines. (incl. Reset?)
Connect the Apple clock to the VIA clock inputs.

One of the Apple clock lines (which one?) ==> PSG clock inputs.

PSG outputs ==> some sort of Amplifiers, as shown in the PSG data
sheets. Maybe put in two pots to adjust levels.

Then it would make sense to also:
Connect the VIA Interrupt lines to the Apple IRQ line (or one to IRQ
and one to NMI?) so the timers can be used in a meaningful manner.
Connect the RESET line to all chips via diodes as you suggested.

Plus of course the usual set of capacitors and resistors, which I
wouldn't know thanks to not knowing enough of electronics. So a
schemacic would be really cool.

-- 

Linards Ticmanis

 The Master said, "The business of laying on the colors follows the
preparation of the plain ground."