Replacing 6550s

From: Ethan Dicks <erd_at_infinet.com>
Date: Sun Nov 22 23:09:26 1998

>
>
> Hi all!
>
> Thanks to everyone who supplied information regarding the differences
> between the 6550 and 2114 (especially Ethan who supplied pinouts).

You're welcome.
 
> I'm wondering if it would be possible to replace the RAM in an old PET
> with a modern SRAM by use of some kind of plug-in adapter.

Sure thing.
 
> An example of what I'm thinking of is the SIMMfonie or AmiFast Zip-to-SIMM
> adapters for the Amiga 3000. Could something like this be built that
> would plug into the 16 22-pin sockets on the PET motherboard, and lead to
> a single modern SRAM? Possibly with the addition of a couple of clip-on
> leads to get two more address lines to the RAM adapter so that the PET
> could have 32K.

There is that big edge connector on the side of the static PETs (it was
two seperate pin connectors on later PETs). All the signals you need for
RAM expansion are there. For maximum preservation, I'd consider pulling
all the 6550's out of the board and sticking them in a bag. Then, wire
up a 6264 (or 62256!) to a small board after finding a suitable connector.
I expect the pinout to be on ftp.funet.fi, but if it isn't, I can dig
out my PET schematics. If I still had my old 4K PET, I'd build one, too.

You shouldn't have to build a clip-on, but if you did, there's nothing
to keep it from working. You can get all the data bits from two sockets,
and all the address bits from one socket, plus the additional bits
from somewhere else. Remember that part of the logic in the PETs decodes
the A8-A11 lines to chip selects. You need the original address bits,
not the decoded ones.

-ethan
Received on Sun Nov 22 1998 - 23:09:26 GMT