<As I recall, the proper name for the Shugart 506 interface became a <proto-standard called SASI, and is a direct ancestor to SCSI. Though <it's a bit hard to recognize the grandparent in the children's faces. No not correct. SASI did become SCSI but SASI is not ST506. ST506 is a raw device interface that is actually a graduate of the floppy interface for 5.25 floppies and closer to that. SASI/SCSI is a higher level parallel bus with arbitration and other features. the biggest difference is that ST506 assumes no intelligence in the drive and SCSI/SASI assumes some intelligence at the slave. SASI Shugart Associates System interconnect The offered that as an interface from the sa4000 drives to a interconnect bus. Others that offered similar boards were Xybec Western Digital and Adaptec. Ampro and others would formalize SASI to SCSI. SCSI Small computer System interconect Used to interconnect intelligent DISKs, TAPES, and CDROMS mostly. It can be used to interconnect systems and offers high data rates. ST506 the raw device (drive) interconnect used initially by Shugart for hard disks and was adopted by others. The basic interface is serial data and the disks data rate on seperate connector using rs422 balanced lines. Control signals like step, direction, head select were on another cable that is bussed liek those for floppies. It's limits were up to 8 heads (later 16) and 5MbS data rate. EDSI was an outgrowth of this. Allison