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From: cybernesto <cybernesto@macgui.com>
Newsgroups: comp.sys.apple2
Subject: Re: Looking for a specific Echo Plus disk
Date: Tue, 22 Feb 2022 17:14:32 -0000 (UTC)
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> Hi, can you clue me in as to how the second AY is addressed on the Echo+
> so
> I can fix the issue?  I haven't been able to find any decent technical
> documentation on the card.
> 
> Cheers,
> -RB
>

Hi RB, the ECHO+ uses the BC2 line to enable one or the other PSG. BC2_1 is
connected to PB3 and BC2_2 to PB4. I actually managed to build this change
and it worked like a charm. I created a pull request. Let me know what you
think or if you need any further info. 
I saw a TODO about finding a hi-res picture of an echo+ card. Was it for the
reverse engineering or for documentation purposes? I can try to scan mine if
it's still necessary. There were two versions of it. I have the mono version
only.