[Coco] coco serial port (Douglas Bell)

[Douglas Bell]

Hi Doug,

 

Thanks for keeping me in the loop.  I plan to subscribe to this list but
haven't yet.  You can forward this information to the list if you want, or
put anyone interested in direct contact with me.

 

The 6502 and 6809 have similar bus protocols so it is easy to interface a
6502 UART peripheral (I think maybe 6551) to the 6809.  I remember favoring
the 6551 over a 68xx UART because the 6551 has a higher level of integration
(internal baudrate generator I think).  In the CC, the 6809 has a companion
chip (6821 maybe).  The mod consisted of removing the 6821 from the board
and replacing it with a socket, installing the 6821 on the daughter card
that we made which included the 6551 and RS232 level translator, and then
plugging the daughter card into the socket.  The 6821 provided access to
address, data, and control lines needed.  I think there was one unused
memory decode on the 6821 which was used for the enable on the 6551.  There
were one or two flying leads to the mainboard.  A thin cable exited the case
to a 4 pin DIN connector.  As far as mods go, it was both mechanically and
electrically 'clean'.  The intent was to add a real serial port for OS9 so
we wrote an OS9 driver which I recall being a straightforward adaption of
the standard bitbanged serial driver.

 

It is probably similar to "the missing chip" article that was mentioned.
The lack of a real UART in the CoCo was an unfortunate omission especially
for OS9.

 

I am not much of a CoCo expert.  I was more of a Model 1/3 expert.  I
completely disassembled TRSDOS for the Model3 (TRSDOS 1.3 I think) and fixed
numerous bugs and added several features including support for double sided
drives.  I think my double side mod even works with Model 3 emulators that
emulate double sided drives.

 

Matt

Mathew Boytim

maboytim at yahoo.com

 

 

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