As far as I can tell, IMI originally used a proprietary interface but when IBM came into the game they switched over to ST506 and IMI drives were in fact put into some AT's; same HDA but a different PCB. I have various versions of the 5014/5018/5021drives and have in fact switched boards back & forth. And yes, the drive clock signals were used for data transfer, at least by Cromemco; once the appropriate sector was located, data was clocked into/out of a shift register by the SysClk signal from the drive. m -----------------Original Message---------------- Date: Sat, 1 Dec 2001 10:28:45 -0700 From: "Richard Erlacher" <edick(a)idcomm.com> Subject: Re: HD Controllers - S-100 & Corvus Omninet I had a couple of IMI drives back in the '80's and find, from my doc's that they were, in fact, ST506 compatible. I wonder if the drives you're seeing in your brocheure are earlier than that. The 8" Quantum drives all provided a clock on the data cable, that could be used to recover data. I'm not exactly sure how this was intended to be used, but several controllers for 8" drives relied on it, not having a PLL on board. The Shugart drives apparently didn't provide this clock. Dick