Re: ECHO+ Card

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

Michael Black wrote:

>Michael J. Mahon (mjmahon@aol.com) writes:

<snip>

>> FWIW, it surprised me when I saw that the Mockingboard used two
>> 6522s to control two 8910s.  Each 6522 has two 8-bit ports, and one
>> would have sufficed.  (Of course, another would have been required to
>> add speech, then.)  It seems that the Mockingboard was simply a
>> "doubled" mono design.  The early models even brought the "extra"
>> 8-bit parallel port of the 6522 out to a header on the card.
>> 
>> -michael
>> 
>I remember at least one article in the hobby computer magazines
>that used that sound generator, and then used more than one PIA
>to controll it.  I forget the exact details, but it struck me
>at the time that whoever designed it did not understand the sound
>generator.  Admittedly it is an odd interface buss, not so useful
>for connecting to the average CPU bus directly (hence the need for
>something like the 6522), but it is intended for connecting to
>some sort of buss, meaning most of the pins can be put in parallel
>with only a separate address pin of some sort (it's been a while
>since I looked at the pinout).  You're right, that board did
>not need two 6522's.

The 8910/8912/8913 chips were originally designed to be
controlled by a General Instruments PIC 1650 processor,
which had unusual bus control lines.  Only two of these lines
must actually be manipulated to control the chip, and with
a little luck, the "handshake" lines on a 6522 might do the
trick.  (Of course, an 8-bit port must be used to source and
sink generator register addresses and data.)

Given the unusual signal arrangement, my guess is that
the original designer took the straightforward approach and
used the other 6522 port for the bus controls, so that any
timing arrangement could be created in software.

BTW, I misspoke regarding the auxilliary 8-bit port on early
Mockingboards.  It was not the 6522 port, but the two 8-bit
auxilliary ports implemented on the 8910!

-michael

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