Some 8" drives, notably those from Shugart, had optional circuitry which you could install yourself, BTW, to do things like recover clock from the data stream, hence the SEPDAT and SEPCLK signals, and mask the sector pulses if you wanted to use hard sectored media in a soft-sectored mode, or whatever. The signal you are probably going to want is the raw data, which is common to all of the various versions. Oddly enough, the half dozen or so SIEMENS drives I have all have this logic as standard. the use of the same signal as both clock and data stems from the fact that FM data has the clock embedded in it. Hence, if you don't separate them on the drive, the controller does that by taking the signals whether they're separated or not, and feeding them to the respective circuits whose job it is to process them. They then use the signals either combined or separated, and process them as they need. These circuits are the things which got many people into the habit of referring to a drive as being either single or double density capable. Without them, they're all capable of either, since the rate at which flux reversals are written on the medium is still the same. MFM simply integrates the clock and data in a way which doesn't waste half the bandwidth on clock pulses. Dick -----Original Message----- From: Bill Sudbrink &lt;bill(a)chipware.com&gt; To: Discussion re-collecting of classic computers &lt;classiccmp(a)u.washington.edu&gt; Date: Tuesday, July 27, 1999 4:53 PM Subject: RE: Cromemco 4FDC, How do you format a disk? <snip>