Re: Ultima 3 "Mockingboard" version.

[mjmahon@aol.com (Michael J. Mahon)]

Linards Ticmanis replied:

>"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.

Exactly.  I don't remember which clock was used for the PSGs...

I DO recommend hooking up the IRQ (I did, but didn't use NMI) for
the reasons you suggest.  It's a simple mod to a Mockingboard
which allows both 6522s sets of timers to be used--for example
for a source of periodic interrupts and/or a real-time clock.
(If you use one counter to accumulate the "heartbeat" counter's
overflows, then you don't lose counts during disk I/O's and other
interrupt disabling activities.)

-michael

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