I'd use perfboard and two sockets, one is a 40 pin wire wrap socket,
the other a regular 28 pin socket.

The wire wrap socket has long pins that will extend through the
perfboard into the existing 40 pin socket.  Then wire everything up. 
Put bypass caps on the adaptor board.

Alternately, get a 40 pin ribbon cable <--> IC adaptor such as
Digikey's c2pxt-4006m-nd :
http://www.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=C2PXT-4006M-ND
Then wire that cable to a PCB with the 28 pin chip.  Upside is that
you don't have a huge PCB hanging off the original 40 pin PCB,
downside is that you have issues with cable length affecting clock and
other signals (put the crystal on the PCB with the 28 pin chip).

There is a lot more you can do with ribbon cable in this area as well 
(two socket connectors placed slight apart make a full IC socket, for
instance).

Good luck!

-Adam


On 1/14/06, John Nall <jwnall@gmail.com> wrote:
> This is a strange little problem, and I'm trying to figure out the best
> way to do it.  I have a little prototype board with a 40-pin socket
> soldered into it (originally there was an 18F452 plugged into it).  For
> an project that I am doing, I want to plug a 28-pin dsPIC30F3013 into
> that socket.  Obviously some pins will go unused, but that is OK.  One
> way, which seems pretty awful, would be to just hard-wire a 28-pin
> socket to the appropriate pins of  the 40-pin socket.  Seems like there
> should be a more elegant way, though.  Ideally, I'd like to do it in a
> manner which will allow me to go back to plugging the 18F452 in when I'm
> done.  Any suggestions?
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist