Well, after much troubleshooting & actually buying a D11-500, I found that my issue was fixed by setting the portspeed value in the config file to 9600, contrary to most of what I had been reading in this thread. So, the DirecTV D12 does work with bxs122's version of the plugin. I have now successfully registered it on a Vista 32-bit MCE box & a XP 2005 MCE box, only having to change the portspeed to 9600 on both to get them working. After I got the D11-500 (with 32-bit Vista MCE) working, I did the same for a D12 (with XP 2005 MCE) and have reasonably fast serial control with both. On both setups, I used the Iogear GUC232A & a null modem cable.
I don't know why the speed had to be reduced for it to work, unless it has something to do with the specific Null Modem Cable I used. I purchased the Female To Female Null Modem Cable found here, mostly because I like the small size and don't want 6-10 feet of cable back behind my equipment. Actually, I doubt that is the issue, since I also tried it with a Radio Shack Null Modem Adapter without success. That was before I decided to try changing the portspeed to 9600.
Anyway, thanks bxs122 for the great plugin!
RHH