Long story short.... I have my BB2 working again. I guess you can say almost any slot machine can be fixed if you're willing to keep buying more and more parts! Someone told me that I might be using an outdated clear card (correct or not), so I purchased a 1851 card to use instead of my 1810 card, which didn't fix it. I then purchased a reasonable reconditioned MPU from a Minnesota company and swapped it out with the original one, but the error still persisted. I finally bought a brand new backplane and swapped it out with the original one, and the machine stopped halting when it got to "loading serial no module failed. / serial no : error registering device". I would like to believe that changing the backplane was the solution, but maybe the final time of reseating the MPU and disconnecting and reconnecting the molex connectors to the backplane made a reconnection that wasn't there before. I'm not that knowledgeable about this machine, but ignorance is bliss and I'm just happy that it works again. I now have an extra MPU for back-up. Although no one was able to help me solve my problem through my post, I'm very grateful for NLG and the extensive archives it maintains which I spent hours reading through trying to learn what the problem might be. They were especially helpful when my bill validator didn't work with the new backplane, and I learned the jumper configuration wasn't set-up on the new backplane for a MARS system.