The older SGI 4D twin towers (professional series) used a SCSI bus that was routed through a connector arrangement up through the smaller drive tower. Unfortunately, I seem to remember that because of the stub length off of these connectors, it violated the SCSI specs pretty badly. My guess is that because the early CPU cards that were fitted in these systems (IP4 and IP4.5) only supported async. scsi, they were more forgiving than a sync. system would be. BTW, I believe that one of the reasons for using ESDI vs. SCSI in these critters was that, at the time, the ESDI drive/controller combo was quite a bit faster than async. SCSI. They intended the SCSI interface for peripheral access only... That said, I did manage to get a SCSI drive working in one, but since I had the ESDI controller and drives, it didn't seem worth it at the time to pursue it further... As far as the twin tower power series, my memory fails me... I don't remember when they began supporting sync. SCSI, but my guess is that it began with the power-series CPU boards. -al- -acorda(a)1bigred.com